To power down most phones, hold down the power button until a prompt appears. Tap on the pop-up message. Some devices may have a "Power off" command in the software settings.

If your phone is crashing or locking up after boot, try rebooting into safe mode. In the previous step, tap and hold on "Power off" until the prompt changes to "Reboot to safe mode." Tap OK.

With the phone in an off state, hold down buttons in a combination to enter the bootloader. Google Nexus models have specific combinations like holding Volume Down and Power simultaneously.

Your device's bootloader may look different from this Nexus 5 screen. That's okay. Note the essential information available to you here: phone model, product variant, serial/IMEI number, SIM and bootloader locked state.