If you have received an email stating "ACTION REQUIRED NOW: If you do not provide these documents, your coverage will be canceled by your insurer." please reach back out to us with the information requested, which may include proof of U.S Citizenship and proof of Social Security Number.
The reason this happens is that the information that you provided upon enrollment did not match what the Health Care Marketplace has on file for you. To resolve this, documents must be provided to them to confirm the information is correct and the full list of eligible documents can be seen here on Healthcare.gov.