Macgawd,
cannas are frost sensitive, so foliage (leaves/stem) will be toast with any frost. Mine in zone 9b are evergreen in sheltered locations (near shed/house/wall..etc) but the ones out in the open get frost burned and new ones sprout from the ground when temps warm back up. Of course we dont get below 30* much, so i dont bother digging the bulbs up, but im sure they can withstand a certain degree of cold temperatures underground. Someone who grows cannas in a colder area will be able to help you with that.

Zone 7 is borderline. You may see new sprouts coming up next spring around May, or they may have frozen or rotted over the winter. A lot depends on the variety of canna, whether they are near a foundation on the sunny side of the house, how well-draining the soil is, whether we get a cold and wet spring, etc. I suggest you wait and see - give it until June just to be sure.