Recovering severely underexposed video
The biggest problem you are facing is that a DSLR typically records to the H.264 codec. That codec tries to minimize the file size and the only way to achieve that is to throw out negligible information.
Sadly human eyes are not sensitive to black parts of video, so dark areas are highly compressed. That means you will have a really hard time brightening your footage, since most information is simply lost.

Had you had access to a RAW or lossless recording codec, you might have been able to recover a reasonable amount of your footage, but that would have cost much more storage space and meant more post production work.

If you would like to learn more about codecs, there is an in-depth explanation on Phillip Bloom's Blog.