You could try holding the foot brake on, whilst moving the stick (doubt it will help).

I once had the selector stick on my spares car. You might be lucky enough to have the same problem.

With the engine off.

If you look down through the selector channel in the gate (center console), you will see a sliding flat flange/spring type set of plates.

With a flat screw driver gently push the locking plate out of the way to release the gear selector. (takes a bit of jiggling around)

Pull the selector all the way down to D and lubricate the sliding plates with WD40 or some such. Work the selector to ensure it is free.

If it is not this then you might have a more serious problem with the selector cable.