If possible to install, sturdy fencing is the best defense against groundhog damage. While groundhogs are capable of climbing, having a fence that also is buried a bit into the ground (which they can't dig under) can help to exclude them from vegetable gardens. Various animal repellent sprays or dusts are also on the market, but their efficacy can be unreliable, and they will need frequent reapplication as they wear off or wash away in rain.

Once the consumption of leaves is blocked, the plants should recover on their own if they have enough energy stored in their roots or remaining foliage. (It might take time for new growth to kick in...possibly a couple weeks.) For plants too weakened or denuded by groundhog damage, replacing them would be more practical to not risk losing-out on crop production time. Otherwise, any routine fertilizer you may already be using on the vegetable plants should be enough by itself to encourage new growth if the plants are vigorous enough, combined with making sure the roots have enough water during dry spells.

Miri