I'm sorry you had to cut the topknot. It takes time to grow back but it will.
I hate it when the groomers put bands in. I always cut them out first thing when I get home and re-do the topknot myself. I had one groomer put Betty Boop's topknot band in so tight that she couldn't blink or close her eyes! Her eyelids were pulled up so high that the whites were showing!
Yes, it was that tight! I always take her bands out at night before she goes to bed.