1. you are out of memory.
2. htaccess file is corrupt.
3. bad server config. idunno what.

solution:
1. reboot your vps, or submit a support ticket @ your host, or add some more ram.
2. delete your htaccess file in public_html and see if it works, if yesfind a good htaccess if no thenrestore htaccess.
3. hire a servernerd