How To Fix HTTP 500 Error

I recently tried to change the theme for my blog but when I applied the new theme it didn’t seem to work as nothing loaded but a blank page afterwards.

Now, whenever I try to open the site, I get a HTTP 500 Error (IE7 gives the error, Firefox gives a blank page).

I obviously can’t log in to change the theme back, so the question is, how do you edit your blog without logging in via the web??? Or is there another solution to restore my blog?

Thanks for your help.