5
Has your server started moving everything over to php5 as many are doing...not that it should, yet unsure just you asking made me remember when i was with illusionfxnet and they were so xcited they just updated then everyones site went haywire and then they told us
Yet check your database with logical table checker really this is a must have in XOOPS core i feel as it tells you of unseen database errors, like missing contexts or repeated entries that can make things vanish like that...or even users not in a group you didn't know about untill installing it a year down the lines and findig that is why your site had been slow...
yet just a sugestion to find the cause...as somewhere between your php and MySQL it isn't fetching the database back again as everything else is showing...