Hard fall on stairs on left buttock with dent and hematoma. Will these resolve or surgery be necessary?

This is a very common condition on te buttocks and breast as result of  blunt trauma (hit, falling, etc) which cause the fat cells to die and in response creates internal scar tissue with an indentation as result. Unfortunately nothing to do to prevent the indentation after the event (trauma, injection or infection). You will know how severe (in your case I consider will be minimum) the damage was until couple of months after.  The options to cosmetically improve the area may be fat grafting, fillers (if it is very small) or if it is very large and deep, then a reconstructive procedure will be needed. Dr. Cardenas 

Without seeing any photos it sounds as though you might want to visit a plastic surgeon now to see if in fact you do have a hematoma and/or intervention is recommended sooner than later. Glad to help. 

Dear Lorena D,it is hard to tell for sure without an examination. In case of fat necrosis, fat transfer is usually required to address the deformity. I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Hi and welcome to our forum!
Following a traumatic event, the development of localized swelling associated with overlying bruising and underlying firmness suggests the possibility of a hematoma. It could also simply represent posttraumatic swelling and bruising without hematoma. 
Seek the advice of a board certified plastic surgeon to determine if drainage is required. An untreated hematoma may be prone to secondary infection and also may leave permanent contour irregularity.
Best wishes...

Hello dear!!
Thanks for the question and provided information as well.
Unfortunately, without pictures, it is very difficult to give you a proper answer. l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)