This happens if the Host management agents are busy or crashed down specially the hostd service. Since only the management agents are down your VM's will still be running and you can RDP in them.
Are you running into any Storage Issues lately..? Did the issue occur after you did a rescan of the storage adapters.?
Select a Host -- configuration -- storage adapters -- devices .. Do you see any dead paths.? If you see any dead paths or any data stores inactive then the Situation what your having here is something similar to an APD.
Check the VMkernel.log and Vobd.log if its pointing out to any storage related errors.