What theme did you try installing? Did you edit anything while doing it?

No editing. Just dragged a theme for another website I am amking about model horses onto the wrong page and it downloaded before I could cancel it. The error seems to be happening to other users now too.

Do you remember where the files were downloaded to (before you deleted them)? I'm thinking that something was probably overwritten.

Templates folder I think?

What's inside the templates folder now?

Hmm, if it was any of the templates from here then just deleting the files should have fixed it. Otherwise, I don't really know since I've only encountered that error when I forget to switch themes before deleting the one I have.

It was a template from the link you sent me with allt he templates. I found one perfect for my other site in the process of scrolling through

Was there an "index" file in the rar/zip the theme came in?
Also none of those can be used straight from the site, they wouldn't work (I should've explained that, sorry). You would have to set it up in the same format that mysidia uses, with its own folder, a template.tpl file and a header.tpl file along with the css.

No index file and if there was, it isnt there.