There are spray products that you can get at the vets and/or some pet shops that are enzyme-based - the enzymes break down the proteins in the urine and therefore remove the offensive smell. You might need a couple of applications but I reckon something like this should do the trick - just make sure you really saturate the area you're treating.

I always keep a bottle of the stuff around because it's handy with toilet training accidents and/or other body fluid moments that the domestic goddess sometimes encounters.