My Joycon won't turn on

This morning I turned on my Switch trying to play a game, but the game would ask me to detach the right Joycon to play. I was surprised, because it was the first time I saw something like that. It turned out my left Joycon wasn't working, so the game wanted me to play with the Joycon held sideways.

After detaching both Joycons I realized the left one wouldn't turn on, even if I pressed L or re-attached the Joycon, the LED would remain off. My Switch also couldn't find the Joycon when "Searching" for it, even when it's still attached, and "Pairing new Controllers" doesn't fix the issue either.

How do I get the Joycon to turn on?

It's easy to forget since Joycons are normally synced by attaching them to the Switch, but they do have a Sync button. Holding the Sync button down forces the Joycon to turn on. Alternatively, pressing the Sync button, then pressing another button, like L can be a bit faster.

The Sync button is located between the SL and SR buttons, next to the LEDs, and can only be accessed if the Joycon isn't attached to anything.