Is an Opposition a pleading that has to be addressed

From time to time, courts misplace documents, some important ones, as in your case. The attorney may not have received the document which you think was served upon him or her by U.S. mail. But, you say that you have receipts. Unless you sent the attorney a copy of the document, return receipt requested, I am uncertain what "receipts" you refer to as evidence of service upon the attorney. Knowing what you can or should do next involves knowing what is the current status of the case. Since the court has located your opposition, is the matter now set for trial? Is the attorney claiming non receipt asking for some kind of delay or other relief? To anwer your question of what to do next the status of the case is of singular importance. If you do not know the status, then check with the clerk of the court, preferably in person.