What if my bandwidth free 100GB gets exceeded?

What happens when my free 100gb gets expired, will my hosted website will stop working via Netlify what exactly happens and also how bandwidth are calculated based on which it is been calculated. Can you be brief on it. Thanks in advance.

Have a look at Limiting bandwidth/traffic to netlify on starter tier plan as I believe posts in that thread will answer your initial question.

Also What does actually count in bandwidth? will likely provide an answer about calculation of bandwidth