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Check the printer cable, if it is directly connected to the SAM4S POS register by a cable.
Re-seat the printer cable, or at least make sure it is firmly seated.
Check the printer as to whether it is Online, and also whether it has a roll in it still, fed in properly.
OK. Do you need basic instructions on using the printer, then?
And, has it ever worked for you before?
Yes, check all the printer connection cables for sure.
A cable may have been dislodged.
If it still doesn't work, try turning the whole thing Off - monitor, printer, POS system - for at least 15 minutes. That may be all it needs, that much down time and cooling off and resetting too.