If Stainless I'd do as suggested and fill it with HOT water and oxyclean and let it sit (Make sure to mix the oxyclean really well so you don't have any particles). If it is aluminum you may want to just try boiling some water in it again to see if that will help. If all else fails you can take a scrubby to it with baking soda, after that though you'll need to re-passivate it by boiling water in it or putting it in the oven at ~300 for half an hour or so.
"People should not be afraid of their governments. Governments should be afraid of their people." - V
Secondary: Shady Lord RIS, Water to Barleywine, Pumpkin wine, burnt mead
Kegged: Crappy infected mild
Bottles: Apfelwein, 999 Barleywine, Oatmeal Stout, Robust Porter, Robust smoked porter, Simcoe Smash