[BadEgg]

Quick fix (not the best choice) - WD40 in the keyhole. Might get the key to work, but will not lube the lock mechanism.
Delayed gratification - Use white lithium grease (spray can w/ straw). Takes a day or two to penetrate, but will lube the lock long-time.

Is the key bent? Go to a VW dealer and have them cut you a spare key using your Golf's VIN. Was the hatch/lock cylinder replaced? You'll need to get that lock "keyed" to so it can open with the same key as ignition/doors.

If you need to get something out of the boot/trunk, fold the back seats down and crawl in there! With some light you should be able to see the emergency internal hatch release and open the hatch from the inside.

Good luck.