I've been able to use Google Maps as normal at home (strong WiFi; 2 bars in 3G) with no problems for ages. Suddenly it says it is in Offline mode so won't plan a route; tapping the message bar does nothing. I understand that offline mode is now available, but why should that disable online mode? How can I get it back online?
It's not the version, and permission to access Internet is on. I think it may have been a temporary glitch: after an hour, it was back to normal & has been ever since, so hopefully problem solved.
You may have accidentally turned off the permission required by Google Maps to access internet.
Applies to rooted Lollipop+ devices: go to Settings -> Privacy -> Privacy guard -> look for Google Maps and grant it the permissions you want.