I have downloaded your manual. You have left out the odd step. This is the procedure from your manual: -
1 Switch the receiver on.
2 While holding down the TONE button,
press and hold the
STANDBY/ON
button for about three seconds.
3 When you see RESET? appear in the
display, press the TONE button.
OK?
shows in the display.
4 Press TONE once more to confirm.
OK
appears in the display to indicate that the
receiver has been reset to the factory default
settings.
If you can not reset the receiver this way, the outlook for it is bad. At the very least the BIOS is corrupted and more likley the DSP board is destroyed.
With a receiver of that age it is unlikley repair will make economic sense, but by all means see if a reapair shop will give you an estimate.
When you have it repaired, or buy a new receiver, include a UPS in the budget.