There's a special mini button labeled RESET right below the microcontroller. This is the, well, reset button. It will perform a clean restart of the microcontroller or let you start the bootloader by hand.

Re-starting/Resetting

To re-start your Circuit Playground (basically, same as turning it off and back on again) - click the reset button once. The Circuit Playground will restart itself.

Bootloader Mode

To start the Circuit Playground bootloader - click the reset button twice. The bootloader will run for 10 seconds during which time you can upload new code. You shouldn't have to do this unless something got a little confused in the 'Play, but its an essential skill! During bootloader mode, the red #13 LED will breate on and off once a second.

It's good to practice both resetting (single click) and bootloading (double click). Check the Power Up test page for more details.