I've had this problem since upgrading from version 3.x (whatever I had before). I now have 4.0.6. (MediaMonkey Gold for life!)
Anyway specifically what happens is when I click on any of the Library/Collections or on my old playlists, they appear to be empty. The first time this happened I just about flipped my lid - I swapped to MediaMonkey because it seemed to handle my 30,000 file database quite well and as you can guess it is a nightmare to imagine recreating those playlists. Anyway I don't know what I did but the playlists at some point magically came back so again this time I am not that worried. I trust that MediaMonkey hasn't REALLY lost everything, but I don't understand why it isn't displaying. I can see the MM.db file in my AppData/Roaming and it is around 140mb which seems the right size. So the database is there, and the files are all there. Oh yes, I should mention after a hard disk failure a few years ago and a miraculous recovery of ALL my music, I am taking no chances and have my music stored on a Network Raid storage device. Anyway the files are definitely intact and they appear when I browse via My Computer/Network/etc in MediaMonkey, just not in the Entire Library. I would have thought the Library list would appear faster than waiting for a scan of the files through the network but that's not what I'm seeing. Actually on closer inspection, the Library isn't entirely blank. The folders listed under Entire Library -> Location are all there. Artist and album names are similarly there, genres are listed, etc. However none of the actual songs that should appear in these categories will display in the details or browser windows.
So, on the offchance it would help, I did a complete database optimisation and the library and my playlists seem to be populated again now. I'm now attempting to close and open MediaMonkey to see if it retains this working state of affairs => but now MediaMonkey is semi-frozen and refusing to close. I can play/pause and adjust the volume but not change the song and nothing actually plays. I am going to have to end the process tree to get rid of it. And now I can confirm that when the program is reopened, everything seems blank again.
I checked whether my AppData folder was wrong in the registry as per another post I saw on the forums here and that was not the problem in my case. The datestamp on the MM.db file is changing and updating as I would expect when I close and open MediaMonkey but something just isn't quite working right, and it loses anything gained by a database reoptimisation every time I am unable to close MediaMonkey cleanly. I suspect if I managed to get a clean close it might clear this glitch, but it just doesn't happen. Now it seems that ending the process isn't even working.
Uh oh, I did another database optimisation to try to recreate my initial success and now it has dropped the database size down to 77mb, which seems far too small.
I am going to restart my computer then try optimising again. I'll report back with progress.
Afterthought: Windows 7 64-bit