Probably a collapsed ignition key barrel, if the key will not turn this is normally the cause.
May, 6, 2012 AT 3:40 PM
I had a 2000 silverado that the key stuck on. But mine stuck in the accessory position.I got the key to turn by removing the passlock cable that is attatched to the ignition cylinder housing. You have to remove the column trim and the cable should enter from the bottom of the housing. Once you get the cylinder back to the off position, leave the passlock cable out, turn the key forward too the on position, and depress the locking pin on top of the housing as you turn the key back to the off position. The cylinder should come out at this point.