Unplug it from the socket and also pull the power cable out of the back of the box.
Make sure the box is switched off (rocker switch on the back) then plug the power back in (plug and connector). Turn the box on. It should go through a boot up cycle. If it doesn’t, either the box or the powersupply brick is faulty - call UPC to replace. Do check that the socket you’re using is working - plug something else in it to check - you never know.
I’ve noticed that UPC have gotten rid of the English option when you call, so if you don’t speak a local language (no idea if anyone there speaks Romansh!) I find the best way to contact them now is via Twitter! Otherwise just keep calling and asking politely until someone answers who can speak English.