My watch is locked in power reserve. Pressing side buttons does not resolve

Hi

Apple Watch displays the time and a lightning bolt (and cannot be used for any other functions) when it has entered into Power Reserve mode, which happens automatically when the battery reaches a low charge level or when it is enabled manually.

To turn off Power Reserve mode, restart your Apple Watch: remove it from the charger and then press and hold the side button until you see the Apple logo. You may need to charge it before it can be restarted.

More information:

Check your battery and charge your Apple Watch - Apple Support

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