Usually when you get a loan from the bank you'll put up some form of collateral such as a vehicle home or other asset.
If you fail to pay back the bank they can then come repossess your car, truck or any other vehicle that you've put up for collateral.
If you've put your home up as collateral the bank can seize your home and foreclose on the house.
They can also take you to court and sue you and you'll have a ding on your credit rating that will prevent you from getting future loans.
Always try to work something out with the bank if you've become unable to make a payment on the loan.
It's better than just not paying and going through all the legal processes that can hurt your credit big time.