Car loan help

I have a car load and defaulted back in march. I paid it in april and then paid april and mays in the month of may. they added april's to the end of the loan (extended it) without telling us and now says we are month behind. they will repo the car tomorrow if we dont come up with the money. Can they do that. How do handle this? Then they said they would garnish my husbands wages if they repo it and we do not get it out of repo. Can they do that? We paid june, july, august and sept. but really we were an entire month behind without knowing it.