The alarm went off yesterday. Why did it not go off today?
Perhaps you switched off the alarm function by setting the control lever to the top position.
Perhaps you set the sound level and/or light intensity level too low. Set a higher sound and/or light intensity level.
Perhaps there has been a power failure that lasted longer than 15 minutes. In this case, the appliance will have remembered all settings except the time. The time will be flashing on the display. Set the correct time.
If it still does not work after re-setting and re-activating the alarm, the product may be defective.
Contact the Philips consumer care center in your country.