What happens is your body was shocked by the surgery. When the body is shocked, it thinks the world is ending, and it shuts down all non-essential functions. Hair growth is non-essential, so it threw the "off switch". The hair stays on your head for about 3-6 months, then will fall out. After it falls out, it takes another 6 months to start growing again. (These are all approximations).
So by the time you are 12 months postop, you should start seeing it regrow. The losses, when caused just by the surgery, should not be permanent. If you are low on iron or some other nutrients, it may have trouble coming back. But you won't know that unless a blood tests shows a deficiency. Do not take extra vitamins, especially iron, because it's possible to take too much.
In the meantime, you could try a shorter hair style that will allow the remaining hair to fluff up more. You can try a wig or hats, or... I wouldn't recommend Rogain.. but I don't know anything about it really.
I've lost at least half my hair, and am almost 7 months postop. It's still falling out, but might have slowed down a smidge. I cut mine shorter so it fluffs up more.