On June 11, 2018 Google started requiring billable access to the Google Maps Platform API. Details on this change can be found here
To fix this issue : 1. Register for a new Google Maps API key here. 2. Enter the new API key in the “Google Maps API Key” field at the bottom of the All-In-One Event Calendar part in the Word Press administration portal. 3. Save the settings on the page.
*Note: We provide links to third-party websites in an effort to help you resolve your issue. We are not responsible for the information on third-party websites and we cannot assist with implementing resolutions from these websites.