Several things can happen.
Wages can be garnished. However, social security is exempt from garnishment.
WI is a community property state -- so they can go after both you and your spouse -- even if the spouse is not on the obligation.
Any bank account with your name or spouses name can be wiped out -- but, again social security money is exempt.
Subject to the exemptions in your state, your home and auto could be attached.
You cannot be arrested.
If you have no income other than social security and you don't own anything, then you are "judgment proof". That does not mean they can't get a judgment. It only means you don't have anything they can take away from you.
You will still be subject to collection calls.