Canterbury bells are a biennial. They bloom the second (sometimes third) year and die. You'll need to reseed. You can collect the seed from the plants. The seed pod is just under the blossom. Allow them to dry. Lots of seed.

Thanks for the info. Do I remove the dead flower and then dry them somewhere?

I will remove the dead petals so they won't look so unsightly, but be careful not to remove the seed head. These pods are prickly. I usually leave the plant in the ground as is to mature, turn brown and dry out.
I love them but haven't been able to grow canterbury bells- deer and woodchuck eat them.