Restart the MAC(power it off and on)
Hold apple + s down after you hear the chime, or if using PC keyboard, hold Window + s down
When you get text prompt enter the following terminal commands to create a brand new admin account (hitting return after each line):
mount -uw / rm /var/db/.AppleSetupDone shutdown -h now
Then turn the Mac back on.
May have to re-assign user permission afterwards using server manager, but all user accounts and file structure is there. The reset only removes the administrator account.