If it is all of the way empty than you will probably have to bleed the air back out of it. If there is still a little in the bottom it will probably be OK. Do you have a leak or something? Where did all of the fluid go?
On the newer models with the hydraulic boost systems the main wheel ports on the master cylinder that go to the wheels are on the bottom of the MC. In the side of it there are two bleed screws for the primary and secondary. These should work to bleed it. Just have someone pump up the brakes like you would if you were bleeding them from the wheel, have them hold the pedal down and then open the bleed screws. It will be less of a mess if you have a small piece of hose to put over the end of the screws to kind of direct the fluid. Just make sure you keep plenty of fluid in the resevoir.
not if the MC went dry like he said. I had to bleed my MC the last time I did my brakes. I bled all four corners several times with no luck before I realized that there must be air in the MC. (The MC never went dry by the way... never did figure out how air got in there in the first place...)
It is easy to do also... just do like stang9300 said!