My guess is that it's beyond hope of regenerating. The curious thing is that it rotted just because it was sitting in a pan of water. My second guess is that it didn't rot for that reason. The rhizome probably had some rot in it to begin with and keeping the soil wet just aggravated it or accelerated the rotting process.

Cannas are generally pretty tough, but I've bought rhizomes that rotted in the ground for no apparent reason after being planted. I have also seen some for sale in bags of coarse sawdust that were not wet at all, but still rotted and dead.

I may be wrong, but in my observations, terrestrial cannas with light-colored flowers (white, yellow, and sometimes pink) seem to be more susceptible to rotting that those with orange or red flowers.

Cannas should not rot just because the soil is damp. Cannas, like much of the banana tribe thrive in warm shallow water, but all plants can be attacked by fungus, bacteria or toxins. I would look for other causes, toxins such as salt, pesticides, too much fetilizer or an infected wound in the rhizome. If there is a firm part of the rhizome left, cut all the dead away, let it dry and dust it with sulfer, then replant it in a new sterile pot with fresh soil.