I have had the same problem multiple times. The minimaps or the main window, or all of them (what made me come searching Geonet) go blank. No matter how you zoom or refresh the page. I closed the browser and came back - still blank.
Ultimately I purged my browser's cache of everything from ArcGIS.com and it worked again. Kind of a pain, but the only way I've found to reliably get back to where you can see the maps (make sure to hit the save button before you leave - even though you can't see anything, it will save your changes).
A man's feet should be planted in his country, but his eyes should survey the world. - G. Santayana