make sure that your theme is in the themes directory... you might have deleted it?
if it's there, maybe you changed a setting to a new theme? you can browse the settings by using phpMyAdmin and poking around the database associated with your wordpress install.
If all else fails, reinstalling wordpress by hand shouldn't change any of your information stored in the database (assuming you didn't make any theme modifications and such...). but just to be sure, back up your database first either through cpanel or phpMyAdmin.. all of your content should be stored there.[hr]
actually, your site doesn't show up... seems like your htaccess is in an infinite loop? you might have tried to block some files and inadvertently shut down access to your theme css files and such...
if you can't access wp-login, you might have moved the root directory or something? you'll need to change .htaccess to reflect this change if that's the case. there's also a setting it wordpress about that.