Besides repossession, what can a bank do if I don't pay the mortgage and the line of credit offered by the bank based on equity in my house? In other words, I've been using the line of credit to pay the mortgage and the line of credit monthly installment. So, what happens once it runs out and I can't pay either?

Normally they will foreclose on the home, the loan agreement on the line of credit will determine if they are allowed to come after you for any balances after foreclosure