This worked for me.
Try rebooting into safe mode:
Reboot to safe mode
If you?re experiencing issues - such as apps crashing, reboots, or fast battery drain - on your device, they could be caused by a third-party app. To check if this is the case, you can start up your device in safe mode, which temporarily disables third-party apps that you?ve installed.
Ensure your device's screen is on, then press & hold the Power button.
Touch & hold the Power off option in the dialog box.
Touch OK in the following dialog to start safe mode.
Try using your device in safe mode to see if the problem persists. If it doesn?t, restart your device to get it out of safe mode and uninstall recently installed apps to see if you can pinpoint the one that?s causing the problem.
I tried this, Voice Search (OK Google) returned; when I rebooted back to normal mode, it was still working. If it isn't working when you reboot to normal, then start uninstalling the most recent apps you've downloaded since the time the problem started.
You can also try wiping your cache from recovery mode.
Go into Bootloader Menu and clear Cache
Hold the Power button until the device powers down completely.
Continue to hold the Power button while pressing the following buttons based on the device:
Neuxs 4, Nexus 5, Nexus 7, and Google Play edition devices: Press and hold Power Button and Volume Down simultaneously.
Once at the Android bootloader screen, press Volume down until you see the option Recovery mode.
Press Power button to restart into Recovery mode. You should see an image of an Android with a red exclamation point.
While holding down the Power button, press Volume Up.
Use the Volume keys to scroll down to wipe cache partition and press Power Button to select it. The device should clear the cache instantly.
Select and press the Power Button on Reboot System Now, then see if the issue is resolved.