having problems with slots

Question

hi. i'm programming in qt and i have encountered a weird problem

when i'm adding new slot i'm getting this error message

:: error: collect2: ld returned 1 exit status

this is the code:

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0);
     void setVisible(bool visible);
    ~Widget();

protected:
     void closeEvent(QCloseEvent *event);

 private slots:
       void setStrings(int value);//this slot or any other definition causes the problem
      void iconActivated(QSystemTrayIcon::ActivationReason reason);//alredy compiled slot. working perfect
};

what can be my mistake??????????????