If it unlocks then it'll just be a case of brute force and ignorance I think. A damn good shaking whilst pulling, then cleaning ans oiling the runners if you're successful.
If it's the lock that's jammed, then spraying WD40 or 3 in 1 oil in the keyhole. If the lock has actually broken the links to the upper and lower lugs then you definitely need locksmith help.
You could try Graham, whose number can be found in this forum somewhere.
