Well it can be alot of things. First did your host change things such as migrate you to a new server. Upgrade software or patches that impact your permissions. Or the worst somebody hacked into your site. Your host should be able to provide you with any activiy on your account, Besides that I would for sure make a complete backup of Tables and files to be safe.
You could check your admin log files in the ubb control panel to see if someone other than yourself was in there changing settings. I don't know if that would really help but it may show someone else was in there other than you.
