Cutting down the foliage likely has little to do with your delayed canna emergence. Most of us have had a rather severe winter with very low temperatures. If you did not protect the rhizomes from freezing they may have been frozen. Zone 7 or zone 8 residents may often get away without providing freeze protection to outside cannas, but winter temperature variations can turn bad.

You might try digging and inspecting for damage.

Kent

I agree with everything Kent said, it's still early, but I would do a little digging to see if the corm is mushy, hopefully they wont be.

If they are hardy to zone 7-8 they will return. Spring might be late this year. I know they are good in 8b-9a