When was the battery last replaced? You may have corrupted programming after the power cut due to a bad battery. Also, has the machine been set to clerk push button login or clerk secret code? This is part of the system programming and should have been set by the dealer and demonstrated to you. There are ways to read programming so that may be the next step. You can read the keyboard layout if necessary and see where your clerk buttons really are.
Registerman
August 2012