I have the very same issue on an iPhone 6s.
Can't exactly tell when the watch and the phone lost connectivity, but I'm unable to pair them since a few days for sure.
I've rebooted the iPhone and even reset the watch (after also trying the latest update from Polar).
Pairing the watch with a Samsung Galaxy works like a charm, but the iPhone doesn't even show a glimpse of a reaction and also the watch doesn't signal that it detected any device.
It used to work for almost 2 years, but now it's dead. And no - I did not change the firmware of the watch until today. Only change was the IOS update(s).
Needless to say that none of the recommendations above helped...