site can't access. - class_core.php on line 317
 

i get email,

Database error in vBulletin :

mysql_connect(): Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)
/var/www/vhosts/site.com/forum/includes/class_core.php on line 317

and site down for several time,now its back to normal,after restart my server.
host support says its because apache eror mail function.
why?

Check your server logs. You did not provide us enough information to assist you.

Apparently your mysql server was down or it quit.

I think the connection failed (which happens before the user/password is checked). It could be that the socket (and/or the host name) is wrong, or if it was working before and you haven't changed anything, it may be that the database server isn't running.

It is not a software issue, it is something on your hosts end, so I would contact them.