This is caused by both the external NiCad battery and the internal back-up battery both being completely flat.
SOLUTION
Provided the internal battery has not been fatally drained, the following procedure should revive the unit.
These steps must be completed in this exact sequence listed.
1. First plug the AC power adapter into a known working and actively powered AC outlet.
2. Then insert the small plug connector of the AC adapter cord into the instrument's charging socket. (You may or may not hear a small beep from the instrument.)
3. Do not press any keys on the instrument yet - Wait for appx 5 - 10 minutes with the unit connected to the AC power source.