Just like any other Administrator or Root password, if you forget or lose the Root password of an ESX server, it could be a difficult situation. This section describes how to reset your ESX root password.
In order to reset your root password for an ESX server, you will need to reboot the ESX server and then interrupt the boot up during the GRUB screen. Once you boot up and reach the GRUB screen, Type in: a
This will temporarily modify the boot up parameters.
Once you press the "a" button in the previous step, you will see a screen like the one shown in the screenshot.
Now type in: single and press enter. This will boot up the ESX service console into "Single User" mode which has the full Service Console functionality but does not load the VMKernel.
The service console will boot into single user mode and then will show a root shell prompt.
At the root shell prompt, type in: passwd
This will change the root password to whatever you type in next under "New Unix Password".
Verify the new password by typing it again. If you get the message "Bad Password: it is based on a dictionary word", you can either ignore it or select a more complicated root password.
You have now changed the root password.
Now, reboot the ESX server so that it will go through a normal startup and start the VMKernel. You can do this by typing in: shutdown -r now and press enter.
Once you issue the previous command, ESX will now begin the reboot cycle. Once it is rebooted, you can use your new root password for administration.