Insurance companies, despite some of their warm-and-cuddly advertising, aren't in business to make your life wonderful, they're in business to make a profit. Sometimes, that leads to dubious, or even entirely unjustified, refusals to pay claims.
This situation has a very simple either-or: either the policy was still in force, and they have to pay the death benefits, or they have to refund all of the premiums they collected after the cancellation -- which they'll have to prove.
Please get a lawyer to help you with this, because it's the only way you can deal with an insurance company on something like a level playing field.