is ESXI host is part of vcenter and you have domain.
have done below step to recover the root password.
Step 1: Connect to host through vCenter server,Need to click on the Configuration Tab. Then select the “Advanced Services” selection from the Software box on the lower left. Then you click on the “Properties” link that is shown in the picture below.
Step-2
Go to Directory Services Configuration window that is shown below. In the select “Service Type” drop down you will need to select “Active Directory”. The in the Domain field We need to type in the name of our domain that we will be connecting to. Next step is to click the “Join Domain” button and We will be presented with an authentication window shown in the next step.
Step 3: In this part We need to enter in credentials that will allow us to connect and join the ESXi Host to the domain. We can enter our credentials in the format listed below (Domainuser) or use this format ( administrator@test.com).
Step 4: After successfully entering our logon ID our ESXi host is added to the Domain. We can see from the image below our host was added to the default computer container since I did not specify another OU for them to be placed into.
Step-5
Now try to login through our Domain credentials .
Step6
We will reset the "Root" password.
Once you reset the password than remove the server from domain.
I have tested and it is working for me ....