When pressing the menu button to the right, you get a menu. There you have to choose the forced flash. In the other settings the flash is not guaranteed to go off. Then while pressing the shutter release button, wait till you see the flash logo, explaining you flash is charged and ready.
Let me know if it still does not work. But I'm afraid if it does not work when you do what is in the manual, you will have to contact a certified repair centre or buy a new camera.
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