Most likely you have a tumbler stuck in the key cylinder...It is fairly common, especially once it gets cold
Pull the key out about a 1/8 inch and take something slightly heavy (small ball peen hammer) and tap the key back in the cylinder with moderate force, this in most cases will unstick the tumbler for the time being.(you may need to do it more than once) however you need to remove it, take the tumblers out and clean it and/or replace it if it is worn bad as it will happen again.
DO NOT start spraying crap in it, that will just make it worse, It needs to be in the run position to remove it so you will need to get it to turn, any other removal method risks damaging the column.

As the key is inserted and removed over it`s life, both the key and tumblers wear, all that crap builds up and they can stick, the other issue is, as the key is inserted it will rub on the cylinder and create a small lip at the slot that the tumblers ride in, same result, tumbler sticks.

Just my

Rick