1.create a bool value in the class to check status of button(clicked or not clicked)
bool IsClicked = false;
2.set IsClicked to true if button is clicked.so add below line within button.click handling method
bool IsClicked = true;
3.add following to form load event handling method.
It starts the timer and shows a messagebox if IsClicked is still not set.
Timer timer = new Timer();
timer.Start();
timer.Interval = 120000;
timer.Tick += new EventHandler((o, b) => {if(!IsClicked)MessageBox.Show("Please click the button");});