Is the headstock locked?..ie in parking position?..if it is wiggle the bars a bit as you try and turn the key AFTER you have...

Spray something like carb or brake cleaner down the barrel etc as stuff like wd40 can just pick up the small particles and form a paste that can make things worse,the cleaners will flush stuff through.

Once you get it free I would spray more cleaner through it,till it runs clear and then get some graphite spray(lock lube) to lubricate the lock properly