Certified GKH Gardening Expert
The best thing you can do is add a pesticide to the soil. This will kill the root maggots and prevent any further damage.
Whether or not your lettuce will continue to grow depends on how much damage is already on the roots, and how many maggots there are. I recommend you harvest as much as you can, as soon as you can, and hope for the best. The really critical part is the steps you take to control the pest to keep it from coming back next year. This article will help with that: https://www.gardeningknowhow.com/plant-problems/pests/insects/root-maggot-control.htm