Just ran 13 miles today staring at the "Loading map" screen, and no way to navigate away from it (that I could tell). Not fun. Fortunately I had the watch set at a 1 mile lap, so I had some idea what pace I was running. I figured it would load eventually, but eventually never came.
I've been stuck at the "Loading map" screen twice now. Both times after unplugging from having the watch plugged into a computer. Fwiw, each time I was able to reset the watch and things came back up normally. You might not retain any recording up to that point, but a reset might have helped you get stats/track/navigate from that point forward vs letting it sit on that screen.