You need to make sure the car is stopped completely. For best results, turn your engine off. Remove any obstructions that might be contributing to the belt being stuck. If the belt is twisted anywhere, try your best to straighten it. When all obstacles are removed, try pulling on the belt. If that doesn't work, sometimes inserting the belt back in, might help to get it unstuck. I have seen really messed up seat belts from old cars, but I was never able to get them un-stuck eventually by attempting the above mentioned. If that doesn't work then the seatbelt may need to be fixed by a professional. I don't know if oiling the belt would help, but I don't recommend it either.
Good luck with your seatbelt.