uMap: What to do if layer no longer has data?

I have a map on uMap with 20-25 layers. One of them is not displaying it’s data anymore. It is visible in the layers list, but greyed out like I’d made it invisible. Klicking on the button to make it visible does nothing, as do all the other buttons. In the “show data” window, it shows zero data in that layer. Does anyone have an idea on what could’ve happend and/or how to fix it? I’d rather not recreate all of that data.

If anyone is interested, the link to the map is below, the layer in question is called “U-Bahn”