1. Try to restart the computer.
2. Press Ctrl + Alt & Del keys on your keyboard at the same time. This should show a menu that will allow you to run Task Manager. In Task Manager, switch to the Applications tab. Highlight any programs with the status 'Not Responding' and choose End Task. You may be asked to confirm if you want to end the unresponsive program, choose Yes. Do this for all programs that are not responding.
3. If you cannot shutdown or restart, press the power button and hold it for a few seconds until it forcibly turns off. Wait a few seconds and turn the power button on again.