Since I don't see it mentioned I'll ask, have you checked the database tables for errors?

None.

My host rolled back php to 5.3 and it seems to be working ok again.

I think they upgraded the php version and xoops couldn't handle it

Hello
As I understand you have an error of 500 on your website
So I must say to you there is an error in your index.php check that There is a problem in your index.php page you have to make a backup of your index page and then make a blank page with the name of index.php if the site run there is a problem in your index page
The errors are some you don't define some of the keyword right ways to check that and then run your file
Hope this will help you
Thanks