Hair removed from hairline by accident - when will I know if it will grow back?

I've had several laser treatments to remove hair on my upper back area over the last three years. I recently noticed that there's hair missing from the bottom of my hairline, which is not something I wanted. (My hairline is normally square in the back--now it looks odd.) I think the laser technician must have lasered too closely to the hairline on a recent treatment. My last session was 4 months ago. When will hair come back if it's going to come back?? Anything I can do to stimulate regrowth??