When medical bills are sent to collection a bad mark will be put on your credit report and you will have a lower credit score as a result of it.
The creditors will then likely try to call and harass you or send you letters trying to get you to pay.
This is why it's a good idea to get health insurance to help cover unexpected medical expenses.
Try and work out a payment plan with the hospital as well if you have a hospital bill that you can't pay off in full.