What happens if I cannot pay back an expensive, high interest loan?

Assuming that you have no valid defense to the loan (usury, fraud in the inducement, minority, etc.), and do not file for bankruptcy protection, you will be sued for the amount you owe and a judgment will be entered against you.  The creditor will then attempt to collect on his/her/its judgment by garnishing your wages and other income, attaching your assets, etc.