Updating from version 4.4 to version 5 let me to a dead phone
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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.