Hi guys, I've had the exact same problem today and the Mrs called Salus support line.
Nothing on the thermostat display screen, changing batteries didn't work!
So the guy on the phone tells us to take out the batteries and leave it for a full ten minutes, before putting batteries back in (must be alkaline he stated) and leaving it for ten minutes.
Imagine me rolling my eyes at this moronic idea! "Ye rite, as if that's going to do anything!"
So imagine our surprise when after doing this it worked and the screen came back on! No dismantling, no switches, no repairing to the boiler just leave it with no batteries in for ten mins then put batteries in for ten mins!
Wonder how many people have purchased a new thermostat with this issue having jot realised!
Good luck people.