Nexus 5 IMEI number gone after updating to Lollipop
There is nothing to worry. Please follow the steps mentioned in this Android Enthusiasts answer.

You just have to flash the images properly. Do a factory reset using the recovery mode.


Volume Down + Power
Bootloader screen will be visible
Go to Recovery
Power + Volume up key (few seconds) and then let go of the Volume up key, but keep pressing Power
Select Wipe Dalvik cache
Select Factory reset