Do you have a second key? Sometimes the key or the lock cylinder wear out. If you have some lock lubricant, or even WD40 in a pinch...try skooshing a tiny bit into the key lock, insert and remove the key a few times, and then try to turn it again a few times. If a tumbler is stuck, it will lock up like this...you'll obviously want to consider getting this fixed so you don't get stuck somewhere uncomfortable in the future.