Internal Server Error: can’t access anything!

I got an Internal Server error, and I can’t access the admin area or the blog. I have no plugins. I was using the K2 theme, seemed to work fine, running over WP 2.5. In the settings, I changed the middle image size from default 300 px to 400 px. Then I navigated to another section of the admin area, just checking things (not changing anything). Suddenly while browsing the admin area I got the error. I can’t access anything, not even login or the blog itself. I have no idea what happened, the only thing left is to uninstall WP (it was a fresh install with only 1 blog entry).

I have another WP 2.5 install in the same server, working fine.

Any ideas? should I uninstall and reinstall? I’m not happy doing this. What if next time it happens when I have lots of content? Should I backup the wp-content folder?

thanks