So I can no longer get into my driver's side door with the key - the key will not turn whatsoever. In fact, I broke the plastic part of the key trying to get it to turn.

However, the door locks / unlocks with no issues if I use the key from the passenger side. So the locking mechanism itself wouldn't seem to be the problem, just the cylinder where the key goes in.

Does this mean it is corroded? It did seem to get worse after a long spat of rain that we had - I'm talking weeks of wet weather.

What is the best way to loosen the lock so that the key will work once more?