Shock hair loss after surgery can be an issue.
Shock hair loss after surgery can be an issue after forehead reduction surgery. It isn't necessarily about the blood flow than the trauma of the surgery. There may also be a genetic component if the hair follicles were already programmed to die as in the case of androgenic alopecia. It is always a good idea to follow up with your doctor for a recheck. You doctor will be able to better assess your condition.
Hair loss after a hairline lowering surgery
I don know anything about you, even your sex. If you are a male and had a hair line lowering procedure and lost hair, then genetic balding which is induced by such a surgery, the hair usually does not grow back. If you are a female, then it might grow back, but not always. See your doctor for more information.
Hair loss after forehead reduction procedure
A few factors may be playing a role for you. If the thinning has started within a few weeks of the forehead reduction procedure, it is most likely due to "shock loss", a common phenomenon particularly in women. If the thinning was already present to some extent before the procedure or if you have a family history of thinning hair, you may be experiencing Female Pattern Hair Loss. I would recommend checking with your surgeon. A good, simple option may be to use Minoxidil, an over-the-counter medication that stimulates hair growth.