What to do if your green card is lost and non-USA passport you cannot find and your N400 interview is in few weeks?

Explain what happened at the interview and then see what they ask you to provide. If you filed a police report after you lost it, provide that as well.

Ask USCIS to re-schedule your interview. Go to the consulate of your country to request a new passport. And file an application with USCIS for a replacement green card. Al this will cost you quite a lot of money, but, if you go to the interview without documents, and USCIS denies your N-400 - then you will have all the same expenses plus $680 for filing a new naturalization application.

The first thing you should do is report the loss of both the green card and your passport to the police in the precinct where it took place.Hoping that you have a copy of the green card, I'd also seriously consider filing an I-90 application to replace the lost green card and simultaneously report your lost passport to the consulate of the issuing country. As a citizen of that country, you should also apply for a duplicate passport. Attend your N-400 as usual along with documentary proof of all the steps you have taken in response to the loss. There is a thriving black market for stolen green cards and should that occur in your card, CIS will be on alert for it as should your consulate on the passport. In the face of your lost green card, so close to your N-400 interview, may give CIS cause to hold off a decision on your N-400 for the maximum 120 day period within which, it has to make a decision on your case for purposes of conducting an investigation on this matter.

You can still go but you need an identification.

It would be best to bring the police report (if you made one). Tell the truth and tell the adjudicating officer that your card and passport have been lost (or stolen?) I usually include a copy of my clients' passport(s) with all pages and copy of the green card with all my clients' naturalization application, so hopefully, whoever helped you with your application did the same and the officer will have a copy. Otherwise, the officer will be able to access his computer, see your green card classification and date of issuance. He can also access all your travel dates (dates you left and date you returned to the US) through something called "USVisit". Make sure to have your state ID and all other documents. If there is a problem at the interview about your missing green card and passport, ask for the supervisor.

File a police report to cover yourself for this interview. If you have your State ID or Driver's License, you should be fine. At the interview, the USCIS will make you sign a sworn statement where you attest that you really don't have the green card in your possession. With this sworn statement, you will be admitted to the Oath Ceremony if your N-400 is approved.