Dear WPMU i used defender to change db table prefix and now

Dear WPMU

i used defender to change db table prefix and now the whole site is down and i an getting this error Fatal error: Allowed memory size of 268435456 bytes exhausted (tried to allocate 524288 bytes) in /home2/hassan88/public_html/KHARAGNY.COM/wp-includes/functions.php on line 4782

please help