From time to time your root account can get locked from either entering the incorrect password or using some automation that uses the wrong password. Here are some quick steps.
Reboot the Photon Appliance
At the Photon OS logo screen press e to edit the grub menu
At the grub menu append the following to the end of the boot loader line to boot into single user mode
Press F10 or CTRL+X to continue the boot process
At the prompt type the following to mount the root partition
mount -o remount,rw /
To reset the root password type passwd and enter the new password
If the root account was locked due to x number of failed logon attempts type to following to unlock it
/sbin/pam_tally2 -r -u root
Unmount the partition again
Hopefully you should now be able to log in with your root account!