Apple Watch won't start

Thanks. Sounds frustrating. If you dont see a green lightening bolt right after you connect to power then there is definitely something wrong. I did find this link to some specific issues related to Apple Watch 5. See if the tips here help: If your Apple Watch Series 5 or Apple Watch SE is in Power Reserve and won’t charge - Apple Support

It did mention to try this:

"To check if your Apple Watch is affected by this issue, place your watch on the charger that you normally use, then wait at least 30 minutes. "

Also, when you press the crown and side button, be sure to hold them pressed for at least 10 seconds to activate action.

Does you iphone still have a watch app and does it show the last OS version on the watch? (Go to General -> About then look at what Version lists).

Explain a little more on the full scenario:

Has it been charged?

If it was powered off, was it turned back on (press and hold side button, not crown)?

when was it last on and was it paired ok?

Model of watch and paired iphone.

See this page;

If your Apple Watch won't charge or it won't turn on