Could be you have the steering wheel locked in just the wrong position, try like answerer #1 said and wiggle the steering wheel and turn the key, too- if that doesn't solve your problem, then you may have a lock tumbler that needs to be replaced, oh and now that i think of it a worn out key might be the cause of this too- if it looks worn down in places, try getting a new key cut before you have the tumbler replaced.