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First of all, don't panic!
Now, have you emptied by any chance the contents of the 'templates_c' directory? If not, I would advise you to do so.
Furthermore, there could be a chance that some file was corrupted along the way, either from your unzipping/untarring utility or by the FTP client you used to upload the theme.
It could also be a problem with the theme itself. Which theme are we talking about? Is it yours, that is custom-made, is it one of the themes available for download?
If it is yours, which editor did you use to edit the theme.html file? Did you use a WYSIWYG editor, like Macromedia Dreamweaver or even worse Microsoft Frontpage?
You see, there are a lot of things that could have gone wrong. If you care to answer these questions, then I could provide more help, I'm sure
