On today's troubleshooting, I faced a very weird problem. vCenter Server services were up and running fine, it's able to connect by vSphere client, but VMs, hosts show gray, and I cannot power on VM via PowerCLI. After went through each components of vCenter Server, I noticed the database size was 230GB, but only ~20 hosts were there. So I asked DBA team truncat event tables and shrink database. Issue gone after database optimization. You may want to do same if you face similar issue.