Keep your fingers cross that it's a simple matter of the shift linkage (the parts that connect the shift lever to the transmission) being out of adjustment.

But more than likely it's time for a transmission rebuild or replacement: as others said, could be clutches ("bands"), leaking seals, failing pumps.

Reverse band failure. Transmission will have to be disassembled for repair.

Transmission probably needs to be overhauled.

make and model helps