My Hue app is suddenly showing several motion sensors that have been working fine for months as unreachable. The LED on the sensors flashes red. I haven’t made any changes recently. I’ve rebooted the bridge and that hasn’t made any difference. Any ideas or should I just get the ladder out and re-pair them one by one?! Thanks. 
have you checked the battery level?
Thanks for the suggestions! Five of my seven motion sensors all became unreachable at the same time so I thought it was unlikely all the batteries had gone flat at the same time! I got the ladder out and re-paired them. All of them were fine after repairing apart from one which went unreachable the next day, but I repaired that again and, touch wood, all the sensors have been ok since then!