Written by Robert -
If you have a vCenter host inside a cluster that crashed for whatever reason, a normal reboot of the host will cause you to lose log files. If you can, it's better to start an SSH session into the host and run the following commands:
Hostd is the agent of the ESX server. Vpxa passes the information to the hostd and the hostd passes the information to the ESX server.
Vpxa is the vCenter server agent. This is the part that is managed by the vCenter server. Any changes you make in your cluster go over this service (modifying a VM for example).
After restarting both services, it's very likely that the server will be up and running again. If not, other measures can be taken accordingly.