[Lynne]

That error is saying you are missing the field lastpost in your forum table. Do you see that field there? That field should not just suddenly go missing. What caused your initial downtime over the weekend? And do you have a database backup from before this problem?

Database error in vBulletin 4.1.5:  Invalid SQL:                  SELECT forum.forumid, forum.lastpost, forum.lastposter, forum.lastposterid, forum.lastthread, forum.lastthreadid, forum.lasticonid, forum.threadcount, forum.replycount, forum.lastpostid, forum.lastprefixid, user.usergroupid, user.homepage, user.options AS useroptions, IF(userlist.friend = 'yes', 1, 0) AS isfriend,                     user.lastactivity, user.lastvisit, IF(user.options & 512, 1, 0) AS invisible                                      FROM forum AS forum                                  LEFT JOIN user AS user ON (user.userid = forum.lastposterid)                 LEFT JOIN userlist AS userlist ON (userlist.relationid = user.userid AND userlist.type = 'buddy' AND userlist.userid = 0);  
MySQL Error   : Unknown column 'forum.lastpost' in 'field list'