NoClassDefFoundError no HibernateUtil

Pessoal,

Estou tentando usar o HibernateUtil e estou tendo esse erro na hora de dar um HibernateUtil.getSession();

Meu HibernateUtil:

public class HibernateUtil {

	private static SessionFactory sessionFactory;
	private static final ThreadLocal sessionThread;

	public HibernateUtil(){}
	
	static {
		sessionThread = new ThreadLocal();
		sessionFactory = new Configuration().configure().buildSessionFactory();
	}

	public static Session getSession() {
		Session session = (Session)sessionThread.get();

		if (session == null) {
			session = sessionFactory.openSession(); 
			sessionThread.set(session);
		}
		return session;
	}

	public static void closeSession() {
		Session session = (Session)sessionThread.get();
		sessionThread.set(null);

		if (session != null && session.isOpen()) {
			session.flush();
			session.close();
		}
	}
}