A Congressional Inquiry is the process by which a Congressman's office contacts another government agency (USCIS, Embassy, etc) on behalf of a US Citizen they represent and requests a reply back to an inquiry. Agencies are compelled to reply back with a response. Congressional Inquiries are often an avenue of last resort when a US Citizen has attempted to get an answer themselves and has had no luck. Often a Congressional Inquiry can get the "gears greased" in many situations and help free cases that are unfairly stuck in processing for various reasons.
To request a Congressional Inquiry regarding your case you should contact your congressman's office directly for more information.