[Death by Mirrors]

Yeah, I counted five in Serra's answer. Of course some aren't definite and immediate death, so in theory I can also say "Superman will die if he lost his power to fly while he's up in the air" or "The Human Torch will die if he's engulfed in flames at the moment". Plus anyone who got their superpowers from radioactivity would've been just as likely to get cancer instead of mad skillz, but I take it as a given every hero first needs to acquire powers for this question to count.

So out of curiousity, what heroes were you thinking of? I'm gonna say Deadman is pretty sure to be screwed the moment he loses his powers, and same for Spawn. Actually anyone who's dead to begin with, because yeah, I do consider still walking around afterwards part of their power (even though that subject can be debated).