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.

Apple Watch - Power Reserve

Power Reserve allows you to continue to see the time on your Apple Watch while preserving your battery life.

Your Apple Watch will automatically enter Power Reserve if the battery gets too low, and you might see next to the time. In Power Reserve, press the side button to show the current time in a 24-hour format. The time will display for six seconds.

While your Apple Watch is in Power Reserve, your Apple Watch and iPhone won't communicate, and you can't access other watch features.

Turn off Power Reserve
To turn your device back on, press and hold the side button until you see the Apple logo and wait for your Apple Watch to restart. You might need to charge your Apple Watch first.

Turn on Power Reserve
If you need to, you can turn on Power Reserve. Press and hold the side button until you see the Power Off, Power Reserve, and Lock Device options. Then drag the slider to turn on Power Reserve.

You can also turn on Power Reserve using Glances. Swipe up on the watch face to open Glances, then swipe left or right to find the Battery Glance. Tap Power Reserve, then tap Proceed.

Source: Apple KB - Using Power Reserve