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
 From the Terminal
 Power button
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 until the power LED turns off or stops blinking.
This should make the XO shutdown.
You can then press and release the power button and the XO should boot.
If none of the above methods work, the last method is to remove all power; take out the main battery and disconnect the external power cable. Wait a few seconds, then put it all back together, and press the power button once more.
 What next
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.
 How do I know it is rebooting?
A reboot consists of a shutdown followed by a startup.
During shutdown the laptop:
- displays a UL safety cautions screen with a series of icons,
- turns off the power LED.
During startup the laptop:
- turns on the power LED, (if the power LED does not turn on you have a problem with power),
- turns on the display backlight, though this may be invisible in full sun, (if the display backlight does not turn on you have a problem with hardware or firmware),
- displays an XO icon and makes a startup sound, though you may not hear it if the volume setting has been changed,
- begins to load the operating system and Sugar.