Updating from version 4.4 to version 5 let me to a dead phone I have a Nexus 5 celullar.
I was asked by the system to update some features. In fact it was to update to a new version: Android 5. I accept the request and give the celullar do its work.
At the last pass when the celullar has to be restarted it went dead.
trying to open it give the word Google but not anymore.
Trying to push the power button + the volume buttom down, appears a dead andorid with an admiration red sign and the text : No Command.
The power buttom + the volume buttom up give the possibility of START, Restart bootloader, Recovery mode or Power off.
None of then give any result.
The system in my PC can't mount the celullar. But recognize is a Nexus 5.
Before I could always connect the celullar to my PC.
Thanks in advance to anyone that can help me.
Otherwise  the celullar (only 3 months!) is useless!
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  Note: Please backup your data before following this.


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This situation is pretty easy to overcome.




Download the nexus 5 image from the google developer forum.
Unlock you bootloader to do this. Note: Results in factory reset
fastboot oem unlock
Extract the downloaded file


Use the flash-all.bat file to flash the new image
If the flash-all.bat failed to execute, extract the contents of image-hammerhead-lrx21o.zip
Flash all the files one by one by using the appropriate command.


E.x: To flash cache.img use fastboot flash cache cache.img




Follow the xda post here for more details and troubleshooting.