Might want to check and see if someone jammed some quarters or something in the reader. When you slide a
credit card all the way into the reader, only 2 of the card numbers should be visible, any more than that and there may be something jammed in the reader.
Also, try powering the machine down for 30 seconds. Power it back up and try a card swipe. There is a communication chip in the controller that receives the serial data from the card reader. When that chip begins to fail, it will not recognize card swipes and powering down/up reset the chip.
And as Randy has stated, the card reader may have just failed and needs replaced...
Keep us posted.