From one of our veggie exerts:

The main stem is elongating and will eventually produce flowers and seed heads. It's possible that if you harvest all of the leaves (assuming they are still mild and tender and not bitter) and cut the main stem right above the babe (crown), it is possible that you will get some re-growth. Older plants growing during very warm weather are less likely to produce new growth.

The surest way to get leaf lettuce to regrow new leaves from the crown is to cut all leaves above the crown at once when leaves are 6-10 inches tall. The leaves will re-grow 1-2 more times.

Here is more information on growing lettuce:

Growing Lettuce in a Home Garden | University of Maryland Extension (umd.edu)

You can try planting some seeds in the early fall for a late crop.

Good luck
Len