There has been no hack of either Walkhighlands or of Leaflet.For people who don't know, Leaflet is the most popular and respected modern javascript opensource libraries for showing maps on websites. Leaflet was originally founded by a Ukrainian and they currently have a Ukraine appeal banner on their homepage.
https://leafletjs.com/Leaflet is used by companies around the world from Facebook to the Financial Times, government websites including the US and the European Union, and by the OpenStreetMap project.
Our map tile server is running normally, so the problem is likely corruption in your browser cache. This would be confirmed if you try another browser or device and find it is working. You can fix by doing a hard refresh (or clearing cache); otherwise the problem will go away on its own when your cache expires. As this has seemed quite common on iOS devices in recent months, I've shortened the time it will refresh on its own to a few days from a month.