The can't connect message is usually a network connection between the 2 boxes. Can you watch on-demand TV on the mini box which is another good test of the mini box's connection to the hub.
If the main box is On will the mini box still not connect, if both boxes are connected by wi-fi make sure they are both on the same 2.4ghz or 5ghz bands.
If you still can't connect the mini to the main box when it's On (not in standby), some people have needed the main box replacing.
I would try a factory reset before calling Virgin because if you do need the box replacing you will lose your recordings.
It's only a partial system reset and not a complete factory reset. You are given 2 options,
Choosing Keep Recordings, means you won't lose your recordings and any planned recordings or series links you've set up, but you will
have to sign into your apps such as iPlayer, Prime, Netflix and ITVX again, also Match frame rate and Standby power consumption will
go back to their default settings if you have changed them.