Did you try turning the steering wheel away from the locked position?
Did you try wiggling the key?
The ignition key tumblers are bad causing the lock to be stuck. You need to get it repaired/replaced to solve the problem.
In the meanwhile, if the key is stuck in the ON position, try spraying some WD 40 into the key slot and try wiggling the key to turn it.
If your vehicle is auto transmission. the steering lock assy has to be replaced as a unit. For manual transmissioon models, the key cylinder can be replaced without removing the steering lock.