I spent a long time tonight trying to figure out how to get my Google Map that worked in the debug version of my Android application to work in the release version. There w
Follow the instructions given here. After creating your project in Google Developers Console, select APIs, you will be prompted with a section of Popular APIs, select Google Maps Android API. Press enable API.(the image below is showing the steps in the new version of the console)

If you're still using the old console, enable the API, by selecting Services, then switch the button ON:
Note:
You don't need to enable the API for the debug version!
If you're map is still not showing try regenerating the key!
(new version)

(old version)
Enter the new API key then Sync Project with Gradle Files!