I've never experianced this problem my self, but have you contacted your host to see if they've upgraded anything, php, mysql, etc.
Or if they perform back ups could they put a back up back on from when it was working.
Also try logging into the ftp for your site and clearing our the cahce and template_C I know this has caused some problems in the past with users logging in and posting with my own site.
I have noticed that it's possible to login to a XOOPS site using one login name and password, and then turn around to another browser on a different computer (or even on the same computer) and use exactly the same login info on the same site, and XOOPS will still allow the second logon.
As I'm working on a site that uses paid subscriptions, how would I be able to control such multiple logins? For example allowing some login names (for a group of individuals) the ability to have multiple logins, while disallowing it for others (non-group members).