If you open the door on the left side of the master you will see a vertical row of slide switches labeled M,1 thru 9 then P.   If you have a DMC3-4, you will find a small pin hole to the right of the number 9 switch. On a DMC1 the reset hole will be on the left side of the slide switches located between the number 4 and 5.

Use a small paper clip (the type without a plastic coating) and bend it so that it can be placed directly thru the small hole.

Push it in ( approx. an inch) and hold it for a 5 seconds.

The display on the master will flash. That will indicate that the DMC master has been successfully reset.

Turn the unit back on.

After you reboot it you will need to reset your clock.   Please see your owner's manual on the correct procedure for doing this.