Received suo moto cancellation show cause notice despite filing all the returns on time.

We have received a Suo Moto Cancellation show cause notice from the GST department. The only pending return is the annual return for 2017-18 for which there is still time. We have been filing all GSTR-3B and GSTR-1 on time regularly.

How do I file an online reply? Do I just mention that the returns are already filed as it has been issued in error? Shouldn't they be able to check under the GSTN if the returns have been filed on not?

Please help. Thanks!