First off what error are you getting, it may be possible to sort it without restoring a database back-up.
The best way to restore a back-up is with ssh and the mysql command, if you are not familar with ssh (or telnet) or do not have access to it you can use this php script to restore your back-up.
This script will not timeout on large files, and should run on any server vBulletin will.
http://www.vbulletin.com/forum/attac...achmentid=8933