Cannot shift back to park from drive

I have a 2000 lincoln navigator, V8 5.4ltr suv.down shifted to drive but cannot shift back to park. so I cannot turn ignition off or crank truck back up. please respond asap. what could be the problem.

Answer from Freeautomechanic.com

Ford/Lincoln vehicles are equipt with an interlock solenoid on the steering column underneath. If this solenoid has failed it can cause this problem. The Brake switch can also cause and this adverse affect since the interlock solenoid is activated by the brake switch. Check to see if brake light illuminates when pressing the brake pedal, if it does then no problem there and it would be the interlock solenoid.

The failure of the interlock solenoid is quite common, and a replacement part is easily obtained from a local Ford dealer. If you are not a do it your-selfer  you may want to call a tow truck and have it repaired at a service center.