This has been asked before.
The Crosscity has been around a number of years. So too has the display, now available in IIRC 8 or 9 different versions that all look broadly the same, but the way they each communicate with the controller differs.
If your display is broken, your best bet is to take the bike back to Halfords as the age of the bike and display will determine which version you'll need. IIRC, a previous member with this display did this very reluctantly, but was pleased to discover his branch of Halfords had a big box full of them, and through substitution they found one that worked.