To resolve the status of your client's cancelled GSTIN, you must file all pending returns from the last filed period (February 2019) up to the date of cancellation (September 2019). You cannot file "nil" returns if there was taxable activity, as this constitutes non-compliance and can lead to further penalties. You will be liable for a daily late fee and interest (18% on unpaid tax, potentially 24% if ITC was wrongly claimed). It is recommended to reconcile the pending ITC and pay all dues in cash via the Electronic Cash Ledger before attempting to restore the registration.