I saw something similar just recently myself, also with v5.1
In this case, I was able to SSH to the host, but it would lock up after a few moments, even when doing something trivial like changing directories. All but two of the VMs were moved via HA, and the VMs left over were not production machines, so we opted for reboot. We ended up having to do this using iLO to power cycle the host, because even issuing reboot command from the console would lock up.
The reboot brought the host back to life and we reconnected successfully to the VC. So far there haven't been any additional problems. We weren't able to recover the logs however, so I can't say what the root cause may have been.