If vehicle has been park on a steep hill and the parking brake has not been applied the downhill force of the car may be preventing the park mechanism releasing, the only way to release this is to take the load off the driven wheels this may be achieved by pushing or towing the car very slightly up hill or by jacking up one of the driven wheels, to release the load.
The next thing to check is that the brake lights are working, if they are not this is probably the problem, as this circuit also operates a solenoid which locks the gear lever in park. This could be a blown fuse but is normally a faulty brake light switch, you can normally take the switch apart and clean the contacts as a temporary repair, but replace the switch ASAP, the switch is located on top of the brake pedal, if there are two switches it will be the one with the thicker wires, the other one will be for engine management.
If this doesn't work you can bridge the connector on the switch with a paper clip (see picture) to release the park mechanism, but the car will not have any brake lights so will be unsafe to drive.
Most automatics also have a park release method for use if the car has is without electrical power, these are normal shown in the hand book (under towing) but normally they have a hole near the gear lever where you insert a small screwdriver to release the gear lever locking solenoid.