Here to keep on keeping on.
If a red lightning bolt is on screen, it needs charged. When charging, it’ll turn green. Note: if watch is drained, it may be a bit before it’s green. If watch is blank or has an Apple Magnetic Charger and red bolt, leave it on for 30m. It should be green by then. If not, and using a Non-Apple “knock off” Charger, try an Apple Magnetic Watch Charger, Apple USB Adapter, and a working wall outlet (NOT a laptop or power bank). If it won't charge or reads “accessory isn't supported”, check for plastic films on either side of magnetic charge pad, and remove it. Clean the back of the watch and unplug and disconnect everything for about 10s. Then plug in and try to charge it. Both are magnetic and “stick” together. You should see the green bolt after some charge time. If not, then force restart (Jeff’s step 1) until Apple logo, then let go. No logo? Try another Apple Charger, Apple Adapter, and/or wall outlet. It’s $159 for a screen fix on a $200 watch. If all of this fails, better to just buy a new one.