I gather you dont know why you computer is shutting down although you haven't said so???????
Is this just a one off occurrence?
If not,how often does it occur?
Have you checked the monitor cable as suggested?
What else have you done to try and fix it?
When does it occur - ie what program or action causes it to occur if any?
Are you able to reboot and enter the system??
Please give us more info
Peter