Butt dent due to fall. How can I fix it?

Just like a dent in a wood desk from dropping a paperweight on it, the "dent" is not removed, the desk is sanded down to the level of the dent and then refinished.When your fall damaged the fat where the bruise was seen, some of the cells died and were reabsorbed, eventually being replaced by scar tissue which further contributed to the loss of fat volume by contracting. Hence, the "dent."Since the dent is caused by loss of fat in that localized area, optimal correction of the contour deformity would be to restore fat volume to the area of the deficiency.Sure, there are other options, like muscle flap, implant, or some sort of filler, but the first of these would cause more of a deformity than the original problem and would not be  considered an appropriate option, the second would introduce a foreign body and would require a (large) scar (even though hidden, this is another sort of deformity that would almost certainly not be appropriate as a reasonable choice), and the third would be expensive and temporary.Thus, fat grafting with your own living fat (usually from an acceptable donor site such as abdomen that will not leave its own deformity if your surgeon were to "borrow" a bit) is the best option with the least scarring and most permanent result.Proper fat grafting requires: carefully-harvested fat that does not damage the cells by laser, excessive ultrasonic energy, or high suction (which boils the fat at room temperature, killing the cells)injection of the fat in small amounts into healthy tissue that provides adequate oxygen and nutrients to the transplanted fat cells. This means tiny tunnels of fat into living tissue, not big globs of fat to simply fill out the dent like Spackle or Bondo.Understanding that about half of the fat transferred, no matter how carefully or skillfully, and no matter how high a percentage of "take" the surgeon claims, will ultimately die and be reabsorbed. The good news is this means about half of the grafted fat survives long-term and will not "melt away." This may mean that, depending on the size and severity of the dent, more than one grafting session may be required.There are no creams, foods, diets, or devices that can "remove" the dent, unless it is so superficial that skillful liposculpture AROUND the dent can lower those tissues to the level of the dent (sort of like sanding a desk when you drop a paperweight and cause a dent in the wood). But in this case, grafting living fat that survives seems the best option.See one or more ABPS-certified plastic surgeons who are experienced in fat grafting, and have your dent examined and options reviewed. Best wishes! Dr. Tholen

Hello dear!!
Thanks for the question and provided information as well.
Looks like you could benefit from a fat transfer to fix your problem, but unfortunately, without pictures, it is very difficult to give you a proper answer. 
The average cost varies a lot depending on the clinic, the country and the plastic surgeon. 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 :)