A couple weeks back, we reported on a widespread problem with OK Google "From any screen" (AKA "OK Google Everywhere") on the Galaxy S6 and S6 Edge. Namely, it was gone. While the hotword still worked from the Google Now Launcher and other third party launchers with such functionality hacked in, most S6 and S6 Edge owners were unable to to use the "from any screen" functionality because the option simply disappeared.
Now, it's reappearing! Yay! A large number of you have tipped us to let us know you're seeing the option back on your devices. I checked my Sprint Galaxy S6 this morning, though, and I still didn't see it - but I did manage to get it back anyway, and it only took a couple minutes.
If you're in the same boat, I'd recommend doing the following: go to your application manager and disable Google Play Services (which will revert it to the factory version of the app), re-enable it, then update Google Play Services back to the most recent version. You should get plenty of Play Services errors to direct you to Play Store page for the app, so doing this should be quick and easy. Then go into the Google app's settings for "voice," then "OK Google detection," and see if the from any screen option has returned.
If this doesn't get the settings back, you can try rebooting your device and/or clearing the cache of your Google app. Finally, if that doesn't work, you can try a full data clear on the Google app itself, which will sign you out of your device (warning: this might temporarily break some stuff), at which point you'll need to sign back in.
I think the Google Play Services "reset" should be enough (since most of these settings are controlled by that app), but if that doesn't work, you've got the slightly more nuclear options above.