It's going down to zero tonight. will my petunias freeze ?
If you can cover them with a horticultural fleece, cardboard, or bubble wrap they'll probably be fine.
Don't know what your lead-up-to weather has been like: prolonged rain could make them vulnerable.
Generally, though, the older varieties (the singles not the big frillies) are pretty tough. I have some self-sown plain and single petunias that are coming up for their third winter in the ground. They're not 'pretty' plants any more but they are still flowering reliably - and coming through our 'gentle' frosts (down to -5C) with only 'if I remember' shelter.