Tcs Joining date

Hi all, I've received an offer from TCS and I've accepted and completed the bgv. They've given jun 13 as joining date, however I told them I can't join on Jun 13 due to some personal issues and asked them to postpone the joining date by another 2 weeks but they're not responding.

Now, what happens if I don't join TCS on Jun 13? Will they revoke my offer or the joining date will get extended automatically?