NetBeans Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: DSA

When I was writing program, everything was ok. But suddenly, I have got an error:

Exception in thread "AWT-EventQueue-0" java.lang.NoClassDefFoundError: DSA
at DSAFrame.<init>(DSAFrame.java:33)
at DSAFrame$11.run(DSAFrame.java:618)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:251)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:733)
at java.awt.EventQueue.access$200(EventQueue.java:103)
at java.awt.EventQueue$3.run(EventQueue.java:694)
at java.awt.EventQueue$3.run(EventQueue.java:692)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:703)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)
Caused by: java.lang.ClassNotFoundException: DSA
at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
... 16 more

In 33 line of my program looks like:

public class DSAFrame extends javax.swing.JFrame {

private DSA dsa = new DSA();

and line 618:

java.awt.EventQueue.invokeLater(new Runnable() {
        public void run() {
            new DSAFrame().setVisible(true);
        }
    });

I don't know what happened. I nothing changed. Only add code to buttons action, but delete after got this error.