I had the unfortunate mishap of a VM dropping offline because it outgrew the amount of free space on the datastore it existed on. On the plus side, it happened in the middle of the night and I wasn't on call :-) The on call guy simply had to reboot the box, and it came back online.
When I got in and looked, the datastore that it existed on had 0.00 B of free space. Not very comforting, considering the fact that there were no alarms on that datastore, and there were several other Production VMs on that LUN.
To fix the alarm state, it was easy. Go into the alarm definition and disable and re-enable it. This will force it to refresh it's states and you will get to see all the pretty yellow and red icons that you've been missing.
This also will work if it is reporting a Usage Above Threshold before it should too.