1. Immediately block the cards.Every bank has a number that can be called 24/7 to report the loss of the cards. Please keep the credit/debit cards numbers handy.
This is a very important step as it will prevent loss of money by fraudulent use of the cards.
2. Go to the bank concerned and apply for a new debit/credit card.
Each bank has a separate procedure for this. Some banks (eg, ICICI) give you an immediate replacement for a charge. Others do not. In any case, the new debit/credit card will either arrive at your home by post, or be can be picked up at the bank, in the course of a couple of weeks.
3. When you receive the card, take photocopies of the front and back of each card and keep them safely. You will need the numbers if you have to undergo this process again!