Since July of 2016, Google API key is mandatory. The key is free, however you need to insert it on your site here:
Once you have entered the API key, your map should function properly. If not, please try to deactivate your plug-in and re-install/activate it again.
Please make sure that the Browser API key is inserted in the Browser API field and the Server key in the Server API field.(see the highlighted part of the image)