Onguard U-lock jammed
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 :-)