try turning your steering wheel hard either way WHILE turning the key, sometimes the steering wheel locking mechanism interferes. you wont be able to turn your steering wheel like your actually turning, just move it and it should let the key turn
steering wheel in locked positon try moving it a little either way and the key should turn. The steering wheel has a locking device that wont let the key turn if the wheel is not free or you have the Wrong key
This could be a number of items wrong. Typically, the steering column bolts break. There is a "knuckle" at the end, you should be able to look under the hood and see where it connects to. Or, your steering wheel could be loose. Either way, DO NOT DRIVE, until you have it figured out.