Java RMI server - server error Class not found

Question

Hi there i have been doing some coursework for a auction system. The server has been working fine until today where there is a error occuring:

Server Error: java.rmi.ServerException: RemoteException occurred in server thread; nested exception is: 
    java.rmi.UnmarshalException: error unmarshalling arguments; nested exception is:
    java.lang.ClassNotFoundException: auction

This si the code where the error is occuring

import java.rmi.Naming;
public class auctionserver {

   public auctionserver() {

     try {
        Auction a = new auctionimpl();
        Naming.rebind("rmi://localhost/AuctionSevice", a);
     } 
     catch (Exception e) {
       System.out.println("Server Error: " + e);
     }
   }

   public static void main(String args[]) {
        new auctionserver();
   }
}

I do have the RMIregistry in the path where i am doing the work. Any help would be much appricated. Thanks