I'm currently trying to unlock my bike that has been sitting on my porch for a while, the key works and turns, and I can hear the lock engaging/disengaging, however I cannot get the shackle to come out. I think their might be dust/sand stuck in the mechanism however I'm not sure how to get it off now to check.
Get some "Creep" or other penetrating oil (ask at Home Depot or Lowes and they'll point you right to it). This is similar to WD-40 but it foams a bit and seeps in everywhere. Apply liberally both at both ends where the U plugs into the bar as well as where the key fits in (key may turn without the tumblers moving completely).
This will likely loosen things up...a bit of patience and a bit of trying to move the parts around will usually work wonders.
If this fails, apply some more "creep" and use a persuader (hammer, etc).
If that fails, you're going to need a hacksaw and a couple of blades.
Good luck! Note: This is karma's way of telling you that your bike likes to live indoors when it's not being ridden :-)