If the system does not respond after rebooting, try the following
1. Try pressing Ctrl+Alt+Delete a second time. In some situations this will cause the PC to reboot properly.
If that doesn't help, move on to step 2.
2. Press and hold the power switch for 5 seconds.
3. Remove the battery and disconnect the AC adapter for 10 seconds.
4. Reinsert the battery and connect the AC adapter back.
5. Restart the system
If the same symptom still occur, you may need to reinstall the OS.