I got this error in a wordpress site http://FreeTamilEbooks.com today morning.
Fatal error: Call to undefined function hybrid_get_prefix() in /home/username/public_html/wp-content/themes/themename/functions.php on line 27
Logged into cpanel of the site.
I checked the functions.php and its code.
It seems the code is fine and it is unmodified so far.
Found that the hard disk quota is filled and that caused these weired error.
Removed some huge files like old backups.
Now the site is working fine. 🙂
Thanks for friends for reporting the issue on time.