This is more of a nooby posts, nevertheless I would still try to share it out to starters who have encountered this issue in their environment.
Got hit with this problem recently where out of sudden my RMS went unavailable and before you know it I got this alert .
A quick check on my Event Viewer shows that I got tons of Event 2115 .
After a quick check on things, I noticed that something is amiss and it proved to be caused by my OpsMgr DB settings. Apparently by default, my OperationsManager DB has a file growth restriction and it seems that my DB hit the limit and thus no new monitoring data could be inserted into the Database.
Just go over and set the File Size to unrestricted growth and that did the trick.