Google Map is just showing map controls with a grey background

I followed the instructions exactly on how to setup a map and I got it working with no problems (this was several weeks ago). However, I just noticed that the map is now just showing the map controls with a grey background. Example:

I am not sure what happened for it to stop working - recent update/plugin conflict etc and was hoping someone here could help take a look at this.

I even tried creating a new map on the same page but got same results...