How do you get the Apple Watch out of Power Reserve mode? I got it into power reserve but can't figure how to resume normal mode. I have tried tapping and pressing the screen and holding both side buttons.
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.