First thing you should always do when you can’t connect to – or if there is an issue with your connection between vCenter and ESXi \ ESX is to:
1) check DNS configuration on the ESXi server and your DNS server that ESX points to making sure you have the appropriate entries
2) Check host files etc in /etc/hosts, /etc/resolve.conf, /etc/sysconfig/network and /etc/vmware/esx.conf files
2) try to disconnect and reconnect your ESXi host from your vCenter inventory, this uninstalls and reinstalls the vCenter agent using FQDN and then with IP address if FQDN didn’t work
3) Try Restarting both the vCenter management agent on the ESX host and the ESX host management agent. Learn how to do this here: http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=1003490
4) If the above didn’t do anything for you, it could be lost connectivity to a LUN, which can cause problems with ESX (less now than earlier versions ESX 2.x), connect to ESXi host directly with VI Client and perform a rescan of your storage adaptors and LUNs.