My Galaxy Note 5 is now doing the same thing. You say "OK Google" and give a voice command and the next screen says "you are offline". My wife's Galaxy S6 still works with OK Google.
If you have access to another WiFi network, check your device connections with that network to see if you have the same issues. If not then it is a router issue and not a device issue.
Welcome to Android Central! I agree with GRIMWALD. Does this happen on both wi-fi and mobile data, or only wi-fi? Do any other Google apps/services have problems as well?
I think I bug was introduced in a recent update. I have the same issue and cannot resolve it.
I've tried some of the suggestions in other forums to no avail so I'm going to wait for a future update.