page not found

I’m getting a “Page not found
The page you are looking for might have been removed,
had its name changed, or is temporarily unavailable. ” error when someone tries to comment on my blog, or go to archives,categories, etc. – basically any internal links on my sidebar other than admin.

I reinstalled a backup and that didn’t fix the trick. I ftp’d the theme again – turns out not only did that not fix it and switching to another theme didn’t fix it either.

I need to switch themes once I get this fixed; I just recently had my WP version updated (thanks to support in here!) and I think it’s not working well with the older theme. Thanks for any words of wisdom how to fix the error.