Go through the following points until the problem is resolved:

1. Check your keyboard and mouse are securely connected to your computer – follow the cables and check they are tightly secured.
2. Check for any power indicators on your keyboard – are there any lights on? If you are using an optical mouse, check there is a red light underneath.
3. Check that the clock on your computer is working – this means the computer is running, and may mean the computer is not completely crashed. Try disconnecting and reconnecting the mouse and keyboard (carefully).
4. Leave your computer alone for a few minutes.
5. If it is still not responding, press CTR ALT DEL simultaneously, then choose the ‘task manager’ option. On the ‘Applications’ tab, you will see a list of the different programs that are running, along with the status of each program – this will show you if any of the processes is stuck. If any of your applications has the words ‘not responding’ in the status, then this is likely to be the problem. If the program with the problem is not a document containing unsaved work, select the program and click on ‘end task’. This will close the problem and could un-freeze your computer.
6. If still no response, press restart on your computer (this is the small button, usually situated close to the main on/off button). If there is no restart button, then you will need to turn off your computer using the main switch, leave for a few minutes, and then switch back on. Please note, if you are working on documents with unsaved work, then your work could be lost by restarting or switching off your computer. 

If you have checked all of the above and your computer is still not responding, then you could have a problem with your computer. This could be for a number of reasons, and you will need to get the problem checked out professionally.