You could use the 30 days trial version of Premiere Pro (most editing programs will be able to perform the following options).
The best way to make the video viewable depends on how badly the video is underexposed. If some areas are literally pitchblack, there is nothing you can do about it.
First, try to use the brightness and contrast effect and raise the brightness till you see a difference. Play around with the contrast value as well.
If that does not work, deactivate or delete the brightness and contrast effect and try adjusting the luminance curve. Try raising the curve, so that minimal differences between the shades of gray (lol) will be increased to make them visible. That way, the video won't look really good, but it might be viewable at least.
Try adjusting the luminance curve to something like this:
(Not really sure if the second one would work as intended in this case, but it's worth a shot)
If that still doesn't work, you could try playing around with RGB curves and color corrections. However, if you didn't see any improvement after performing those steps, I don't think you'll be able to perform these steps.
If one of the above steps works, you might be able to even get some color in it using RGB curves and/or increased saturation, but I'm not sure if this will do anything if your material is really that highly underexposed.