Hi,
There is an excellent video on YouTube by GOODTIMES TECH that shows an identical issue with Huawei watch which shows the problem being related to a plastic piece that is worn causing the power button to not activate the switch.
The audio isn’t very clear but very easy to follow and shows successful remedy for the button issue.
You can find it here:
https://m.youtube.com/watch?v=e-_h8R2IXcY
If you’re battery isn’t dead this will more than likely be the issue with your watch.
cheers