Can't connect to ESXi with vSphere Client

There can be several causes. Your first place to check is the VMware KB Article.

After that...

From root SSH on the ESXi console:

services.sh restart

Or "Restart Management Network" from the F2 console.

If you continue to see issues, do what @TCampbell said and remove/readd.

If you are still seeing issues, RDP to your vCenter server and verify connectivity from the server to the host. Check port 901/902 from your vCenter server with telnet <ip address> <port> or another utility.

Did your hostname change, too? You may have to regenerate SSL keys with /sbin/generate-certificates.sh

If you can't connect to it from the vSphere vCenter Server, you should remove that ESXi server from your inventory and re-add it.