|Sorry, but to be able to help you we will need a LOT more information about what happens than it "just shuts down unexpectidly"|
What PC do you have (specs if you know them)
What are you doing on the PC when it shuts down (if it turns on)
When did this start happening?
Have you made any recent changes to your PC?
Do you have an up to date antivirus and have you performed any scans recently?
Sorry if none of that makes sence, and if it doesnt then it might be a good idea to take it to a PC repair shop but...without this information we wont be able to do anything