The answer to your question can be found here:
https://support.toshiba.com/support/viewContentDetail?contentId=2737864
* Power down the device.
* Unplug everything - mouse, keyboard, monitor, etc.
* Make sure the AC adapter is plugged in.
* Press and hold the zero key while powering on the computer. Keep holding it till you see the recovery warning screen.
* Pick your OS.
* Follow the remaining prompts.
The drive will be wiped clean when you do the factory restore. If you don't want to do that, do not proceed.