This can be caused by the selected flash setting. If Flash Mode is set to Autoflash, the flash may not fire if the camera determines there is already enough light to create a proper exposure. Set Flash Mode in the Fn menu to Fill-flash to force the flash to fire every time you take a picture.
NOTE: The flash may not fire even if it is set to Fill-flash when a certain camera mode is used, for example:
Multi-Frame Noise Reduction
Burst / Multi Burst shooting mode (Handheld Twilight)
IMPORTANT: On some cameras the built-in flash needs to be pulled open into the upright position before it can be used.
NOTE: If model-specific information is required for how to pull open the flash or how to change Flash Mode, refer to the operating instructions supplied with the product.
If the issue is still unresolved, service may be required.