I had the same issue after changing my domain password today. I could no longer log into vCenter in any form with my domain account.
Here was my problem/fix:
When we upgraded to vCenter 5.1 I setup Active Directory authentication for Single Sign on by adding an indentity source.
After your fill in the path to your primary and backup domain controllers at the bottom there is a spot for username/password to test the connection. Well during the setup I used MY domain account credentials to do this connection "test" instead of using the service account we created in our domain for vCenter.
So I logged into webclient via the admin@System-Domain account, edited my active directory identity source to reflect the domain vCenter service account (which has a much longer password expiration date than our regular user domain accounts) and presto everything worked again.
Hope this solves someone's problem!
Cheers