Perform the following if the system is no longer responding
1. Press CTRL + ALT + DELETE keys simultanuously twice.
If that doesn't restart the system, refer to step 2.
2. Press and hold the power switch for 5 seconds.
3. Remove the battery from the computer and disconnect the AC adapter. Wait for 10 seconds.
4. Reinsert the battery and connect back the AC adapter.
5. Switch on the computer again.
If the above steps do not solve the problem, you may need to reinstall the system.