Unfortunately, I've struggled with this before and there's no great solution. If your camera captured darkness there's no way to flip a switch and bring the light in. It's ridiculously frustrating; luckily I've only had to deal with it when shooting stuff for my own family.
One bit of advice (I can't give specifics because I work in a diff editing suite): mess with EVERY filter you possibly can, color correction, brightness/contrast, lighting effects, exposure, gamma, etc. You may not be able to totally "fix" the footage but you could also arrive at a happy accident that looks kind of cool and at least makes the footage more memorable. I once dealt with footage that kept going from really bright to really dark and it was looking like crap. I adjusted the frame rate to make it a little more jagged, upped the contrast and turned on a trailing filter. It didn't make the scene any more understandable, but it got compared to "Fear and Loathing in Las Vegas," which may or may not be a good thing...