hai provato a tenere premuto il tasto accensione per almeno 10 secondi? a volte così si risolve altrimenti devi usare questa procedura per per tentare di fare un ripristino alle condizioni di fabbrica da telefono spento:
1. Turn off your phone.
2. Press and hold both Volume Down + Volume Up keys and turn ON Nexus 5 by pressing POWER. You will see the word Start with an arrow drawn around it.
3. Press Volume Down twice to highlight Recovery mode.
4. Press Power to get into Recovery mode. You should see an Android robot with a red exclamation mark.
5. While holding down Power, press Volume Up.
6. Use the volume down or up keys to scroll to "wipe data/factory reset" and press Power to select it.
7. Scroll down to "Yes - erase all user data" and press Power to select it. All data will be deleted!
If not working repeat the steps above.
LG Nexus 5 hard reset