Hi Rania. This use to happen to me all the time. Your computer is overheating, so it has to shutdown so a fire doesn't start. You need to make sure the computer has some breathing room so it can circulate air.

Also, you might want to take it to a local computer shop and have them put a more powerful fan in it so it will circulate more air quicker.

Pulling it a little away from the wall seem to work good for me too. If that is not the problem, then the only other thing I could think of is that power supply doesn't provide enough power for all the parts inside. Basically each part uses a certain a mount of power. For example, your CD drive would use lets say 10 watts (not sure, just guessing).

So you add everything together and whatever it comes out to, the power supply has to be that powerful at least, or more. If it isn't then it will be able to start, but after a little while it will shut down.