Welcome to the Community - So to understand the issue - your vCeneter crashes and you reboot when it somes back up some of the ESCi hosts show up as disconnected - som questions to understand what is going on:
- Is your vCenter machine virtual or physical?
- Do you know why your vCenter server is crashing?
- How many hosts do you have in your environment? When the problem occur do all the hosts become disconnected or just some of them?
- Are you able to ping the hosts that are disconnected from other machines on your network?
- How is your physical network configured? Are all the machines plugged into a common switch?