You've probably lost your memberslist.cgi file in an out of space condition. Please ask your host to ensure that you have enough disk space, then head over to UBBCentral.com and grab the "Rebuildmems for 5.x" script... follow the instrutions in the zip file.
If the rebuild script fails to bring things back to life, you'll want to open a support ticket - the link is in my signature.
After this is resolved, you'll want to move up to the latest release... 5.45c is over three years old...
--
Charles Capps
UBB.classic™ Developer
Having problems? Open up a support ticket!