After experiencing some issues with my Nexus 5, I had to access the boot menu to reset the phone, and clear the cache.
Hold down the power button and volume down button while the phone is off. It will boot to a system screen. Hit the volume down and up button to choose the menu you want and press the power button to select the right one. You'll see a "No Command" icon, then I believe you press the volume button or both power and volume to get to the next screen and you should be able to perform more advanced tasks to troubleshoot problems. There are plenty of forums with more details about what you can do in this mode.
Cheers,
-Aaron
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