I have sometimes come across this problem if either:
A) The number of requests per day for your google-key are exceeded
B) I try to use the map without first reloading the document after reopening (don't know why this happens, should make no difference, but sometimes it works just to reload)
C) Your request towards google fails based on security restrictions, firewall, port blocking, etc. Make sure to open up for any outgoing requests, and that it does not require login.
So far I've tried three different environments(work, home, customer) and they wall work fine after adjusting the security settings.