If your "root" user was locked just wait up to 900 seconds to get unlocked.
Or if you don't have any important VMs: reboot your hypervisor, this will unlock your account, too.
I have this problem, too. Some chinese hackers are brutforcing my root account all the time so it gets locked every half hour 
Workaround: very very high secure password for "root" and user "root2" for your daily doing. They don't bruteforce user "root2" (in my case).
Hope to help
Yes, rebooting hypervisor means downtime for your VMs.
So better just wait for the 900 seconds and retry your login without rebooting
Even if the root user is locked out on SSH, VAMI... you can login into DCUI which means on the "console". Only login over the network is not possible but on the Host directly it should work.
For me the main reason is always the same. If we replace the Host Hardware on customer sites, often we have to reuse the IP and Hostname. With preinstalled ESXi from HW Vendor and more strong password rules from ESXi these passwords doesnt match to the old one at first. But most customers running countless monitoring tools and use the "root" for that so the account locked after a couple of minutes. It took some time to figure this out
Regards,
Joerg