Hey there,
Power off your machine and disconnect all peripheral devices attached to it. Peripheral devices include external HDDs, USB sticks, external speakers, USB keyboards/ mouse, etc. Do not disconnect the AC power adapter.
Now, start up your machine and press the F2 key repeatedly for a few times. Your keyboard LED should flash first and your computer will load in to the BIOS. Navigate to the Exit menu using your keyboard keys, and select Reset Defaults, or any other similar option, and then save your settings and exit.
Hope that helps.