Everything's gone black

I changed a material of a pawn to a simple blue colour and everything just went black. I reverted the material back to the default and nothing has changed.
Literally everything in the editor is now black and building has done nothing to fix this.
There was a message about texture streaming pooling over but I had just started and it loads up more complex maps fine. The message has also gone now but I’ve no idea how I can get this back.

Anyone experienced this before?

Here’s what it looks like. As you can see there is barely anything in it!