If you have not come across this yet, you probably will soon if you have deployed a vCenter Server 5.5 Appliance. The root account on the vCenter Server Appliance 5.5 will lock out 90 days after deployment, or root password change. Not to be alarmed, VMware added the security best practice to encourage administrators to frequently change the password or be locked out every 90 days.
The KB article 2069041 – “The vCenter Server Appliance 5.5 root account locked out after password expiration“ is located here, and describes the process to a) prevent a forced lockout every 90 days and b) how to unlock a locked out root account on the vCenter Appliance.
Certainly a handy KB article if you are approaching 90 days following your vCenter 5.5 Appliance deployments.