Hey there, I ran into this issue before...but it only occurred when my rekordbox laptop couldn't communicate with my cdjs (loss of power, d/c cdjs/switch/router/wifi)
Having more details of your setup would help but do the following.
Shut down everything and unplug everything (hub, etc).
Turn on your laptop and go into rekordbox like you usually do.
Turn on your "hub" and wait for 2 minutes.
Connect your laptop to your hub (assuming its wireless from the look of your picture)
Once connected to your hub, connect the cat5 cables into your one/two cdj.
Turn on your cdjs and give them about 1 minute.
YOu should now see the LINK icon in rekordbox and if you click on it (as you mentioned, it worked before) it should work now.
If the following does not work, try changing the cables that are uses on the cdjs.
If that does not work, check to see if something is wrong with yoru hub power adapter.
If that does not help, try it with a different hub.
I cant help any further because I am not sure what kind of hub you have. Is it a switch or a router? There is no such thing as a hub unless you are speaking of a usb hub which doesn't work with usb..only routers and switches.