You could have a ruptured badder, however I wouldn't believe the gauge until you make sure the pipe nipple to the pressure switch is clear and also the gauge connection is clear. Deposits will close off that pipe and make the pressure switch do funny things. Then dump all the pressure down stream and check the bladder pre charge pressure. If it is zero you can try and recharge it. Open up the drain valve while you are doing it to see if air is bypassing the bladder.