Грешка в HSSFWorkbook

Здравейте, получавам грешка, когато се опитвам да използвам работната книга на HSSF. Вижте тази грешка

Exception in thread "Thread-13" java.lang.NoClassDefFoundError: org/apache/poi/hssf/usermodel/HSSFWorkbook
    at digicare.tracking.serial.BulkUpload.UploadProgress$1read2.run(UploadProgress.java:95)
    at java.lang.Thread.run(Unknown Source)
Caused by: java.lang.ClassNotFoundException: org.apache.poi.hssf.usermodel.HSSFWorkbook
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.net.URLClassLoader$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(Unknown Source)
    at com.sun.jnlp.JNLPClassLoader.findClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)
    at java.lang.ClassLoader.loadClass(Unknown Source)

Ето моят код:

try {
    file = new FileInputStream(new File(FilePath));
    try {
        workbook = new HSSFWorkbook(file);
    } catch (Exception e2){
        JOptionPane.showMessageDialog(null, "Error1" + e2.getMessage());
    }

    //HSSFSheet sheet = workbook.getSheetAt(0);
    //HSSFRow row;
    //HSSFCell cell;  
} catch (FileNotFoundException e1) {
// TODO Auto-generated catch block
      JOptionPane.showMessageDialog(null, "Error1" + e1.getMessage()); 
} catch (IOException e) {
// TODO Auto-generated catch block
      JOptionPane.showMessageDialog(null, "Error2 "+ e.getMessage()); 
}

Изглежда, че когато се опитам да използвам частта от работната книга, тя връща грешка