Reboot
There are several ways to reboot the XO, that is to halt the running software and restart the machine. This page clarifies some of the differences
From Sugar
To reboot the XO, the menu for the XO at the center of the Home view mode in the Sugar UI has a Restart command.
From the Terminal
Alternatively, in a Terminal Activity or Console you can simply type
reboot
If that doesn't work you need to be root or to type
sudo reboot
Power off
If neither of the above methods work, perhaps your XO is "stuck".
You can power off the machine by holding down the power button for several seconds. Details? This should make the XO shut down. You can then press and release the power button and the XO should boot.
Advanced information
See Cheat codes for various buttons you can hold down during booting. For example, the '✓' (check) game pad key forces a more detailed display while booting.
Pages in Category:XO startup describe the firmware and software that runs on an XO during startup.