Debtor's prison was abolished 300 years ago. People don't go to jail for not paying their bills, and so-called library "fines" are not true fines, they are just rental charges. I would appear in person at the time and place where you are supposed to pay, and explain that you don't have the money and therefore cannot pay, but you don't contest the amount nor your liability. It would make no sense for the library to sue over $120 if you don't have it anyway, and have no income. Your library privileges will be suspended and your credit rating, if you have one, may be affected.
