Remove the tire and tube the rest of the way. You can usually do this easily by hand, but if you have trouble getting the edge of the tire completely over and off the rim you can use the tire levers again. Once the tire is off, pull the old tube out of the tire. You can then either discard the old tube or attempt to patch it.
If your tire is flat because of a puncture that came from hitting a road hazard, after you remove the old tube check the inside of the tire thoroughly to make sure whatever caused the flat -- glass, a piece of metal, etc. -- is not still lodged in the tire to play a nasty trick on you and pop a second tube.
