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Unplug the cash register in question, from the power.
Leave unplugged from power for at least 20 minutes or several hours perhaps. Or longer.
Then try it again. Also try turning the manager key to the RST position for at least 15 seconds, then back to REG position again.
Let me know how it goes, please. Thanks.
How old is the register? how well has it been treated (poor treatment includes odd or bad conditions e.g. humidity, moisture, and cold or heat... and storage for a while of almost any kind...) ?
OK. It may be 'broken', given the signs it is producing, and all efforts so far to get it to work again, and given its age and usage.
That means it will need either service-repairing, or replacing. The choice is yours, of course.
Let me know if I can look up local Samsung register repair service co.s, or get you other information helpful to this unfortunate case.