How was it acting prior to this current no-shift situation? How many miles, service history? Any slipping during driving, or delayed shifts?
Did the RPM's previously ever rise during the 2-3 shift?
What does the fluid look like, dark and burned or red/pink? Fluid level?
If all of a sudden, maybe a broken accumulator piston, if gradual, then possibly the 3-4 clutch pack.