I am having the same problem. Initially when I received the 'Smart Hub 2' and 'Wi-Fi Disk' everything set up with ease. However the 'Wi-Fi Disk' disconnected from the network at some point (Nothing has changed from the initial set up, nothing moved, added or changed) and now will not reconnect. I have tried resetting the Disk and starting afresh by connecting directly to the hub using an ethernet cable and the disk responds by turning on a solid blue light after a couple of minutes however when I disconnect the ethernet cable the light turns red and flashes no matter how long I wait. I have gone through the set up procedure multiple times with the same end results
You say nothing's changed, maybe not that you're aware of.
There was a software upgrade recently that put some peoples hubs into Wireless Mode 2 for some unknown reason, this in turn prevents the discs from connecting to the hub by wireless.
Go into Advanced settings, Wireless and check Wireless mode is set to 1
Wireless mode in 1