I hit my nail with a hammer. it became black -purple. what should i do ?

Major pain?: If it's very painful, you may have a hematoma (collection of free blood) that's causing pressure, which could be evacuated by a healthcare provider. If there's not much pain, and swelling is minimal, there's not much to do except perhaps put something protective over the fingertip while it's healing.

When: If it just occurred and the blood under the nail is about 1/2 the size of the nailbed then normally a physician would use a heated needle to poke a hole in the nail and relieve the pressure but if this happened yesterday it is is too late and will just have to resolve (may make the nail fall off). Good luck.

Need more info: Next time be more selective and hit only metal nails. All kidding aside it depends on how hard you struck your nail. If there is a great deal of pain there might be a fracture. If the blow to the nail did not cause a fracture them you have a hematoma under the nail. If the hematoma raised the nail it should be drained by your physician. Best case is no fracture & minimal hematoma. Leave it alone

No treatment needed: Most of the time, a blunt injury to the fingertip, like hitting it with a hammer, causes a "black and blue" mark beneath the nail. This represents some bleeding into the finger tip from small blood vessels (capillaries). It can be very painful. Most of the time no treatment is needed and the discoloration will fade away with normal growth of the nail.

Bruised nail: Hi, nothing needs to be done, there is old blood under your nail which would be absorbed slowly, if you have pain then cold compress, tylenol (acetaminophen) could help.

Depends: If you have acute pain due to blood under the nail, a hot paper clip works perfectly: do not attempt without experience. ER people do it routinely. The nail will be discolored and grooved until the affected area is shed off by growth of new nail. A complete fingernail is replaced in 6 months, a toenail in 12.