If you could recall, was there a specific action that you made before the map screen went black? Like, for instance, were you at the house before going to the map, or did you opened and closed the options menu?
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I tried it twice, first I fully explored the home area (all the rooms and the barn), took Joey with me, and then clicked the car to enter the town map. The map screen was completely black, but I could see that Joey was in my party based on his icon on the bottom left, and moving my mouse to the top shows my inventory as normal. It also let me access the options, which let me restart the game (without refreshing the page) so I tried again.
The second time I went straight for the map without picking up anything in the home area, and got the same black screen while the inventory/options button still works like before. This time I clicked around randomly in the black screen , and I saw the path to the pharmacy highlighted during one of the clicks (simply moving the cursor around didn't highlight anything), and I was even able to go to the pharmacy from there.
That's when I refreshed the page, and this time the map screen showed up without issue. I should note that this could be a problem with my browser (Opera) since I've noticed some HTML5 games, including some of my own games, lags horribly at times, which causes issues that doesn't show up when playtesting from the engine. I didn't notice any lag when playing your game though, so maybe it's something else. Hope this helps!