Hopefully it won't happen again. I am a developer of a maps app for the watch, so I use maps on it a lot and I have only ever seen it once. It seems to get into some state where the map just won't appear. I tried rebooting both the phone and the watch but to no avail, so I eventually re-paired it and that fixed it.
I wish I could say that it will definitely fix the problem and that it probably won't happen again, but I can't. All I can say is that I use it a lot and it's only happened once for me, and that re-pairing it fixed it. Hope that helps.