Fixed it!
Thanks to Somnium86 in a year old post.

1.) went to 'Blocks Admin.' and selected 'FAQ' as the show blocks visible in setting. (FAQ is the only page that the block would appear in).

2.) I now had 'Main Menu' as an option, and I clicked 'Edit' under the 'Action' column. That page presented me w/ a dropdown list of pages that the block would show up in...I chose 'All pages'.

Problem solved; but I can promise you that I did NOT get in the settings that deep and change it prior to this problem....which scares me. Hopefully this is just a fluke. At least I know how to fix it should it happen again.

Thanks for everyone's posts.

You rock somnium86!

I had the same problem, except my main menu was hiding in "Faq". Never occured to me to look for the block in the different sections.
Cheers!

Thanks for the post, it's about two levels above my paygrade though.
What do you mean by "yourprefix_tplfile table"? I have access on the back end to all these files and my path usually looks something like this:

/var/www/html/modules/system/templates/blocks

Where amongst the madness would the tplfile table be? I've searched for a good while and the nearest thing I could find was /var/www/html/modules/system/admin/tplsets but none of the code in it looks like what you referenced.

Let's back up a bit and rethink:

Problem: Main menu missing on front page, is no longer listed as block in blocks admin.

Whatever caused the problem was done through the admin menu within Xoops. No back end files were messed with because the person who caused this error had no access to the ftp side of things.

Question:
The blocks admin section lists all the available blocks. What file generates this list? Let me start there and see if main menu is even listed. Currently, the only place I see mainmenu is:

/var/www/html/modules/system/templates/blocks/system_block_mainmenu.html

and this is the same file I see on other installations.

I can also access this through template manager and the same code shows up and previews fine with workable links. It just doesn't end up in blocks admin and I can't turn it on!

I hope my descriptions are satisfactory. I tend to be to verbose.

Thanks again for all those helping here.

Sorry for the responding delay. I needed a break before my head exploded.

To answer your questions, I'm using XOOPS 2.0.10. I know it's an older version, but it was the newest at the time and it has all the functionality that the site needed for me to be happy.

Also, yes, I'm viewing the blocks in "webmaster" view, so it should be showing them all, but, alas, no "main menu" any more.

I did end up with this error using the debug once I figured out what I was looking for:

Warning [Xoops]: Smarty error: unable to read resource: "db:xoopsheadline_block_rss.html" in file class/smarty/Smarty.class.php line 1084

This obviously explains the lack of my headline content, though I don't know how to interperate what's wrong.

While eventually I'd like this fixed as well, my biggest concern is still getting my main menu back!

I continue to be appreciative of any help. I'm obviously not a programmer and I barely understand how XOOPS works. I knew enough to do updates to the site but as I said, one of my helpers tried to add a custom block and in the process somehow removed the main menu. I removed the one that he added...could he have somehow renamed "main menu" and then I deleted it thinking that his custom one was just cluttering up my view?

Thanks again.

Yes, checking the group permission was the first thing I tried. Main menu is checked. (it says (id5) next to it) Strange that it shows up as a checkable box, but it isn't listed among the rest of my blocks in block administration.
Even stranger that it does appear on the server at
"modules\system\templates\blocks\system_block_mainmenu.html"
Is this the location of all the blocks listed in block admin?
Sorry, getting ahead of myself again.

Could you clarify more about phpdebug for me? I turn it on and I get a list of queries. Is this what you want posted? The front page looks the same to me after turning it on. Same with module/block debug. With this on, I get a new set of queries for every page I visit but nothing flashes "error". i just don't know what I'm looking for.

Thanks.

And I am willing to give admin rights to the site to anyone who would be willing to help. i'm sure this is simple.

May be getting ahead of myself, but I also went to "modules\system\templates\blocks\system_block_mainmenu.html" on the server. It's code matches what is in that template file, the one that does preview properly.

For giggles, I saved that file to my computer and deleted it from the server. I went through the previous steps again and updated the system module. The text after hitting update does say
Block Main Menu updated. Block ID: 5
Template system_block_mainmenu.html updated.
Template system_block_mainmenu.html recompiled.
but when I checked the server that file did not recreate itself. Should it or am i misunderstanding?
I copied the version from my computer back to the server.

Thank you very much for the fast and easily understood directions. Unfortunately, it did not work. When I updated the system module, it did indeed read in that text screen that follows that the "system_block_mainmenu" was reinstalled and recompiled so I got excited. Nothing changed though.
I also went to the template section and there was nothing in yellow and no generate link option. "system_block_mainmenu" is listed there however and when I select to view it the links do show up in a preview screen (although horizontal instead of vertical) but the links work.
Should I still attempt to replace the file? If it helps I'll end with the code found in the "system_block_mainmenu".
Oh, I also just noticed that while my headlines section header still shows on my front page, the content of it is missing. Any chance these two are tied in together?
Thanks a bunch.
~Christian
(contents of "system_block_mainmenu" in template set)
<table cellspacing="0">
<tr>
<td id="mainmenu">
<a class="menuTop" href="<{$xoops_url}>/"><{$block.lang_home}></a>
<!-- start module menu loop -->
<{foreach item=module from=$block.modules}>
<a class="menuMain" href="<{$xoops_url}>/modules/<{$module.directory}>/"><{$module.name}></a>
<{foreach item=sublink from=$module.sublinks}>
<a class="menuSub" href="<{$sublink.url}>"><{$sublink.name}></a>
<{/foreach}>
<{/foreach}>
<!-- end module menu loop -->
</td>
</tr>
</table>

The main menu of my website www.meckyr.com (allowing access to the various sections like news, photos, and polls) is suddenly missing. I noticed that in my blocks administration section of the admin menu that the "main menu" block isn't even listed anymore.
I gave another gentleman access to this site to help with news updates and he did make a mistake or two in attempting to create some new blocks (after I told him not to touch that section!), but would it have been possible for him to delete the "main menu" block? None of the other system module blocks even offer a delete option.
Thanks for any help.
Hopefully someone can just send me a main menu file and tell me where to upload it.
Thanks again!
~Christian

Thanks for the response. Actually, I've never (to my knowledge) messed with the templates. I went to System Admin - Templates, click on list by System and none of the templates was highlited in yellow. I did notice the following file name
system_block_dummy.html
"Dummy template for custom blocks or blocks without templates"
And on a hunch I clicked view and it is empty. Is there code that should be in this?

bump!