comment réparer “HTTP Statut De 500 Servlet exception a déclenché une exception”

j'ai eu ce erore et je ne sais pas quel est le problème
je veux juste vérifier le nom d'utilisateur & mot de passe, puis entrez une page qui montre la connexion ou pas

package cse;
import java.io.IOException;
import java.security.NoSuchAlgorithmException;
import java.util.Date;
import java.util.Random;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

@SuppressWarnings("serial")
@WebServlet(urlPatterns = {"/LogIn"})
public class LogIn extends HttpServlet {

@Override
public void doPost(HttpServletRequest request, HttpServletResponse response) 
    throws IOException {
    String username = request.getParameter("username");
    String password = request.getParameter("password");

    User signedInUser = User.login(username, password);

    if(!signedInUser.equals(null)){
         HttpSession s = request.getSession();
         s.setAttribute("signedInKey", (new Date()).getTime() + (new         Random()).nextInt(999999999));
         request.setAttribute("user", signedInUser);
         String url = "http://localhost:8080/java_project/test.jsp";
         response.sendRedirect(url);
    }

    String url = "http://localhost:8080/java_project/notLogIn.jsp";
    response.sendRedirect(url);
}

}

et c'est l'erreur

HTTP Status 500 - Servlet execution threw an exception

   --------------------------------------------------------------------------------

type Exception report

message Servlet execution threw an exception

description The server encountered an internal error that prevented it from fulfilling       this request.

exception 

javax.servlet.ServletException: Servlet execution threw an exception


 root cause 

java.lang.NoClassDefFoundError: org/hibernate/Session
ir.ac.shirazu.cse.LogIn.doPost(LogIn.java:25)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)


 root cause 

 java.lang.ClassNotFoundException: org.hibernate.Session
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1714)
org.apache.catalina.loader.WebappClassLoader.loadClass(WebappClassLoader.java:1559)
ir.ac.shirazu.cse.LogIn.doPost(LogIn.java:25)
javax.servlet.http.HttpServlet.service(HttpServlet.java:647)
javax.servlet.http.HttpServlet.service(HttpServlet.java:728)


   note The full stack trace of the root cause is available in the Apache Tomcat/7.0.40 logs.


    --------------------------------------------------------------------------------

    Apache Tomcat/7.0.40

plz aider , je ne comprends vraiment pas quel est le problème