check if it is happening with the VMs of just 1 host or many hosts?
If it is just one host then right click>disconnect>reconnect will do the trick
if that is not helping then right click>disconnect>remove and then add it back to the cluster
[NOTHING WILL HAPPEN TO THE VMS INSIDE THAT HOST DONT WORRY 
]
If it is happening on VMs of all hosts then it is a problem with the vcenter.
If it is happening only on VMs of few hosts then find out whats common to all these hosts and that should point u in the right direction. Also check for the availability of the free disk space for these VMs