There is a really annoying bug in Android that makes your Home and Recents buttons disappear and prevents the notification shade from working. It only happens after flashing an OS update without wiping, but since I've now run into this issue at least 3 times after updating my Nexus devices, and it's a pain to find any info on how to fix it online, it's time for a quick post.
Specifically, I just flashed the updated LPV81C L preview build on top of LPV79 (again, I did it without wiping data - just open the flash-all script and remove "-w" to do so) and observed the Nexus 5 boot into this:
If the buttons disappear for you for the first time, you will be stumped. Confused. Mad. You will then try to reach out for your notifications, but you won't be able to. And you will likely just wipe data to fix it. But there's no need.
The fix was found by the founder of MoDaCo Paul O'Brien back in 2013 when the same thing happened to the KitKat OTA. There is only one step, granted that you've enabled USB debugging and installed adb on your computer.
Fire up adb and run the following command:
adb shell am start -n com.google.android.setupwizard/.SetupWizardTestActivity
The phone will display the Welcome dialog where you select your language, then ask you to join Google Now, and finally go back to the home screen. At this point, your notification shade will work. The buttons, however, may still be missing. Don't fret - just reboot.
Ah, much better, isn't it?
Cheers to Paul O'Brien once again for root causing the bug and finding the missing piece of this puzzle.