What to do about a thumb injury from hammar?

Ice: Ice decreases swelling, bleeding/bruising, and pain. Unless it was a very large hammer or you are exceptionally strong, the chance of a fracture is minimal and the pain and swelling will improve in a few days. The black and blue under the nail may take several months to resolve. If there is a lot of pain and pressure under the nail the blood may have to be released by your doctor.