I just spent an hour on the phone with Apple support going through several of the suggestions above. My watch has been in a drawer for a year and a half and paired fine with my older iphone 6. Now my series 1 watch will not pair with my iphone 8.
The support manager's only suggestion was that it must be hardware - I have to prepay $218.31 to mail it to them to fix it.
What else can I try to get the watch to upgrade? It is working fine.