Zygoma reduction has caused volume loss and face to sag, would a facelift fix this?

While a facelift might help, it will not compensate for the loss of volume.  If you go this route I would suggest adding a filler such as Voluma, Restylane Lyft, or Radiesse in the area of the zygoma/midface.

Even giving general advice to your important question would be very difficult without photos and knowing precisely what was done in your initial operation. Many good options are available and finding the one that best suits you requires a consultation with your original surgeon or another recommended surgeon. Best regards.

This is a complicated question, as there are many variables to consider.  It is possible that re-suspending the fat/muscle/skin of the midface is all that is needed.  Revisit your concerns with your surgeon, or seek an in-person consultation with a plastic surgeon specializing in facial surgery in your area.

A facelift is not usually going to fix a sagging tissue problem after cheekbone reduction surgery and it may actually exacerbate it. You need to add some cheek support back to help lift up the tissues. Whether this is best done by reversing the cheek osteotomies or cheek implants depends on what a 3D CT scan shows of your cheekbone anatomy currently.

Unfortunately there are no pictures. If you have good quality skin you may be a candidate for fat injections into the cheeks. If you have good soft tissue thickness and droopy cheeks you may be a candidate for either a Endoscopic cheek lift (which I helped to pioneer) or a high SMAS facelift with or without fat injections. The former operation is performed through lower eyelid incision. I hope you find this information useful. Please watch the enclosed video.

Given your description, most likely it would.  If you are able, you might want to consider returning to the original surgeon who performed the zygoma reduction and discussing the situation with them.  Redoing the zygoma and placing it back in its original position may help, but sometimes the effect of the surgery is to re-drape the tissues and may not be 100% related to just the zygoma position.  Always consider a second opinion in these situations as your situation is somewhat unique as zygoma reduction surgeries are a less common procedure.  

 It is difficult to give advice without a photograph or a clinical examination.  But the reduction of your zygoma may have produced midface ptosis due to detachment of the overlying soft tissues.  This could possibly be fixed with a midface lift.  This would be done through intraoral and temporal incisions.  Find a Board certified surgeon who does frequent facial rejuvenation for the best outcome.  Good luck.  Donald R Nunn, M.D.  Atlanta plastic surgeon