How to fix the rss feed on my site

Have you tried:
– deactivating all plugins (yes, all) to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s). Don’t forget the ones in mu-plugins. If you can’t get into your admin dashboard, try resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– switching to the Twenty Eleven theme to rule out any theme-specific problems.

[Edit] Your feed actually validates but is full of code from hopfeed.