I assume the key goes in all the way but won't turn.
Don't get mad. I've heard of this happening before but... are you SURE you are using the correct key? ???
Okay. That being said. Remove the key and spray WD-40 into the keyhole. Next, insert the key and move in and out a few times to lube the wafers inside. Then insert completely - make sure the handlebars are not keeping it bound up - turn the key.
Hopefully none of the wafers in the lock are broken or bent.

Good luck.
