If your conditional green card expired, and you did not submit an I-751, you are now illegally present in the U.S. Do not apply for naturalization. You are not eligible. Time to schedule an appointment with an experienced immigration attorney. Please see
Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
You still need to apply to get a new/replacement green card (I-551), Lawful Permanent Residents are supposed to have their green card with them at all times. Alien's (or immigrants) are eligible for citizenship after 5 years (3 if still married to their U.S. Citizen spouse). When you apply for citizenship you are normally required to furnish a copy of your green card (unexpired) with your application. You should meet with an attorney who can help you determine whether you want to apply for citizenship.
This answer should not prevent you from speaking to a lawyer and disclosing all facts about your case. Additionally, it should not prevent you from filing to see if you are eligible. Please seek an attorney for an initial consultation to review all the specific facts of your case.