Someone probably put in a different password.
To get it back to default, UNPLUG the 816 from the wall,(there is 110v AC in there) take the cover off, look for the battery on/off controler which could be a small sliding switch or a jumper connecting two pins of center and on or center an off. DO NOT mess with the Earth/Ground switch, this will not help anything, just leave it alone.
A small sign says BATT about two inches up above the power supply and about two inches from the right side.
Flip the switch to off or take the jumper off. Wait 30 seconds. Flip the switch on or put the jumper on the center and ON. Plug the 816 back in.
The system is in default and the #20xxxx1# will put you into programming and return the display to date and time. #20xxxx# will set the clock for AM/PM format.
This is very old manufacture discontinued stuff. Usually the batteries will not hold programming for very long. You can hook up two 12 volt 7 amp batteries to the socket or solder in a new 3.6 Ni-Mh battery to the motherboard.
