My Apple Watch series 3 is stuck in power reserve. What can be done?

Do you mean Low Power Mode?

Ensure the Watch is more than 20% charged

In Settings, Battery, try to turn it iff there.

If that does not fix it or you meant something else...

First force restart the Watch

i.e. press and hold both the side button and Digital Crown for at least 10 seconds, then release both buttons when you see the Apple logo.

Then, although it may seem irrelevant, Force Restart the paired iphone   Force restart iPhone – Apple Support

 Then try Battery Settings again.

If is is still misbehaving Contact Support.

Click here > Contact - Official Apple Support

or phone numbers 

Click here > Contact Apple for support and service - Apple Support