If I can't pay a medical bill what can they do about it

Garnish your salary, attach your bank accounts, put a lien on your home, etc.

These are all possibilities depending on the amount of the judgment and where you live.

If it is medial bills try working out a payment plan or if it was a hospital bill
And it is need based then you could apply for charity. Most hospitals have a charity fund but if it is a medical bill from a dr's office then try working out a payment plan