transmission on 1990 Ford Mustang GT

just today i tried shifting my automatic transmision into reverse and it woudlnt grab neither would parking,but all of a sudden parking grabbed and reverse wont now,what should i try to do?or what could it be?

The first thing that should be checked is the shift linkage and cable to make certain the valve body main spool is properly engaging its internal detents. Otherwise, the transmission might be between ranges.