I had appendectomy my scar came open what do I do?

See Your Surgeon. Wound openings and wound infections are not uncommon with appendectomies. Wounds opening up may do so because of many reasons primarily due to infection, non-healing, or more serious issues. Please see your surgeon promptly for evaluation.
Call your surgeon. It may be infected. Get in to see your surgeon as soon as possible. Most likely it will not require further surgery, only wound care.