What does the (server) error.log say?

I don´t get anything on your website except that it is unavailable.

What happens if you directly access user.php or some other file?

Set your site to the default theme in the database

MAKE A BACKUP (if you havent done so) now and lock it away. (database and files)
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in case you got to take the road down to rebuilding:

- create a new folder
- have a subdomain point to that folder
- install the same XOOPS version there
- copy tables from the old to the new site and test
- install the theme ... test
- install module by module; copy tables; test each one after the transfer
...
see at which point it breaks ...

as odd as it was... i just had the host change the MySQL server and it took a while but i think the problem was resolved. thats all i can think of that i did, and its back up and running once again. If it wasnt the issue then it went down for a couple days and now is back up on its own.