Stamped H1B, Employer cancelling the job offer


I have a valid stamped H1B visa for company A, employer is asking me to join company by end of December. But due to some personal problem i cannot join it. I am asking for extension but employer is not agreeing to it.

If I don’t join will it mean my H1B will get cancelled? what will happen to my H1b?

Thanks & Regards

Kamal Garg