How to fix a computer that is stack up?

How to fix a computer that is stack up? My friend has a problem to her computer. She told me that when she open her computer and gets into the desktop it suddenly stops and she moved her mouse but nothing happen. The mouse stop too. She restart her computer to fix the problem but unfortunately it stops again.
So she called me up for a help and told her the same thing that she did, that's the only way I know. She asked me why her computer stack up or stops whenever she open it. What is the problem of her computer?
How to fix a computer that's stack up?
