I usually just provide lawn fertilizer for my crapes. You could use some root stimulator right now and again in a couple of weeks and maybe again 2 weeks after that to help with the loss of roots. You can get it at any of the box stores. Follow th instructions. Then when you see some leaves you can fertilize lightly with regular lawn fertilizer. Crapes are indestructible.

Mulch is probably the best thing you can do for it right now. I usually use lawn fertilizer as the sole fertilizer for everything but it's still too cold. When you see CM starting to leaf out, it's probably a good time to fertilize.

Now there is a very good chance that the place where your Crape Myrtle was dug up will send up shoots. Those shoots transplant well in fact they usually do better than your large tree. Their roots go out just under the soil and sometimes you will get a lot of stars to plant all over or give to friends. The last time I moved one I ended up with over a dozen starts which I was able to find homes for quickly.