Does a insurance co. have to notify you when a group plan has been canceled because premium was not paid?

I was not informed that my dental insurance was canceled on April 14th until July 27th, because my employer failed to pay premium. I was employed until May, and because my portion of the premium was deducted on my check I assumed I had credible coverage. Before having work done in May, the insurance company confirmed coverage to both me and the dentist by telephone and the claims were later paid. Now they say they made a mistake and are demanding reimbursement. My employer is now bankrupt and I am unemployed. It doesn't seem fair that they can wait over 90 days to tell me my policy was canceled and then hold me responsible. Does anyone know what the law in Illinois is?
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Do check with the insurance commission to be certain what your rights are. I know you usually sign a form at the dentist or doctor offices that says you will be responsible for any portion of the bill not covered by insurance, but these are unusual circumstances.
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