My 2 year old Iwatch 6 just starting doing this last week for the 2nd time. The first time I was able to hard boot the watch by pressing both the crown and the side button simultaneously to force restart. After starting the latest update last week it hung up again on this "BootLoop" and I've done everything short of a hammer.. (Pressing, Hitting, Freezing, draining..) To no avail.. I went and bought another watch and this time I'll keep insurance on it.