To save an underexposed photo there are a few different things we can try from working with what we have to converting to black and white and more.
Before you give up hope and assume that your shoot is a complete failure there are a few things you can do to try and salvage the underexposed photographs that you've taken.
I'm going to be presenting you with three techniques that you can use to ger the most out of what you have. However it is important to note that every photograph is different and will present its own unique set of challenges. These three ideas will give you something to try in the event that you do get home and find your photographs have taken a turn for the dark side, but they might not be able to bring them back, sometimes they truly are too far gone.
Save what you can get rid of what you can't. By simply adjusting the basic settings a bit to expose for the sky in the photograph I create a simple silhouette of the trees and leave it at that. The final step was to clone out the little bits of the run down shack that were peaking out asking for attention, but not adding to the photograph.
Use Graduated Filters and Adjustment Brushes until you can't use them anymore.
Underexposure will happen, no matter what you do. But with Lightroom, using graduated filters and adjustment brushes you can bring the color out in the photo. Or usually you will have to switch the image over to Black and White.