How do you get the Apple Watch out of Power Reserve mode?
Solution 1:
You can bring your Apple Watch back to normal functioning mode by pressing and holding the side button, the same way that you do when you wish to power it on.
This command will only work if you plugged the Watch in, or if there is sufficient battery juice available.
Solution 2:
You can hold the Digital Crown and Side Button at the same time for 6 seconds and then wait until it shuts down and turn it back on. This is force shutting it down and won't damage or effect your watch.