If your situation is very bad, you must apply surgery to correct your droopy eyelid. This kind of surgical procedure can be called blepharoplasty. Surgeons will lift your eyelid, tighten up your levator muscles and improving your appearance. But I am curious about why you get a punch? A punch to the eye is very dangerous. You must avoid such kind of action in future. Hope this helped!
Well, if your symptom is mild, they will recover several weeks later, you needn't apply for any special treatment. And some kinds of eye exercises may strengthen weak muscles and correct your droopy eyelid. Or you can wear a pair of special glasses which have a crutch attached that can hold up the droopy eyelid.