If you lose the root user password on the VMWare VCenter Appliance then you can follow this process to reset it.

1. Reboot appliance
2. When the splash screen for the Photon OS is displayed press the "e" key to access the GRUB menu
3. On the line that ends "consoleblank=0" append the text " rw init=/bin/bash
4. Press F10 to save and boot
5. When the command line is display enter "mount -o remount,rw /"
6. Enter "passwd" then enter a new password twice when prompted
7. Enter "umount /"
8. Enter "reboot -f" to restart the appliance

The next time the appliance is rebooted it is will then boot normally - not to the shell.

More information on the VMWare Knowledge Base at: https://kb.vmware.com/s/article/2147144