You will have to pay the penalty with fees, a maximum of upto Rs. 750 for Electronic Clearing System (ECS) debit bounce on your HDFC personal loan account. You should take ECS very seriously because an ECS debit mandate is just like a cheque issued by you. This means, you need to ensure you have enough funds in the account, so that you get your ECS cleared. 

If you bounce an ECS, you will have to bear the same penalty as you would have for a bounced cheque. And this could be upto Rs 750. 

Keep in mind that your bank runs an ECS and if there isn't enough funds, the bank may run an ECS again at a sometime later (usually a couple of days), that is after a few days. But, if you fail to have funds by that time, your ECS will eventually bounce. 
And also remember that re-run of an ECS is not your right but dependent on your bank's policy. But you should avoid this in every case as credit scores get affected and your creditworthiness will take a hit. Your credit score doesn’t just impact your borrowing, but can even impact your work life, as many companies check credit scores before hiring new employees. So, it’s highly recommended that you maintain sufficient balance in your account to clear EMIs and avoid ECS debit bounce.

Additional Reading:HDFC Bank Personal Loan Emi Missed