I have had balloon thermal endometrail ablation done 5 yrs ago. can my uterine walls grow back?

If you bleed, yes.: Sometimes the gland grow deeply into the muscle and are not completely destroyed. In that situation bleeding can resume later. Remember, ablation was developed to be a quick alternative to hysterectomy, and no guarantees can be made as to cessation of bleeding. If it works be happy. If not, there is probably something like adenomyosis which causes failure and can't be treated with ablation.