Couple of questions: These will help to understand the scenario exactly
1) Are you trying to access vSphere client locally from vCenter or remotely?
2) Is it fresh vCenter installation or did you perform any upgrade recently from 5.x?
3) If you are accessing it remotely - share the OS Version (32 or 64 bit?)
4) If you complain about Inventory is empty - SQL DB is reachable but is it healthy (performance wise)?
5) All the vCenter related services are running?