Modifications can cause them. Crashed tables can caused them. Tempermental servers can cause them. Too many reasons to list.... You need to look at the error to figure it out.
what's the error say?
view the source if you don't see anything at first
PHP Code:
Database error in vBulletin :
mysql_connect() [<a href='function.mysql-connect'>function.mysql-connect</a>]: Access denied for user 'webkinzc'@'localhost' (using password: YES)
/home/webkinzc/public_html/forums/includes/class_core.php on line 311
MySQL Error :
Error Number :
Request Date : Tuesday, December 16th 2008 @ 05:46:45 PM
Error Date : Tuesday, December 16th 2008 @ 05:46:45 PM
Script : http://www.webkinzcoast.com/forums/?do=main&username=forums
Referrer :
IP Address : 24.61.147.135
Username :
Classname : vB_Database
MySQL Version :
-->
Well, the details need to be correct in the config.php file in order for vbulletin to work, so I would guess you should look in there - /includes/config.php