Can you put the front end of the car on jackstands so that you can get underneath? You will need to deterine if it is the shifter, shifter linkage or transaxle. I am assuming that you can't even move the shifter when the engine is off and you have verified that the clutch is uncoupling the engine from the transaxle. Is that correct?
From below the car, you can grab the shift stub at the aft end of the transaxle casing - where the two shift cables are terminated - and try to move it with your hand or pliers. Someone may have to press the clutch pedal in so that you know the engine is uncoupled from the transaxle.
If you can move the shift stub, the problem is upstream and is either a shift cable, cable clip or shifter handle problem. If the clutch is depressed and the stub can't be moved, the problem is internal to the transaxle. I can't get more specific because that was not much information to go on.