After using the gauntlet to snap everyone back, Bruce's arm is badly damaged from the gamma radiation. Before Steve travels back to return the stones, Bruce admits that he tried to will Nat back to life along with everyone else. But, like Gamora, her death occurred outside the power of the stones and thus cannot be reversed. We're not sure where Bruce will show up again, but Mark Ruffalo is already tapped to appear in one more MCU film, so we'll definitely be seeing him again. The real question is whether or not he'll still be in his Professor Hulk form.