There may be few reasons for that

1.Virus

2.Your Ram is going to get damaged, or is malfunctioning.

Install latest version of antivirus software(If you dont have retail version you can try with trial version), scan your computer.If it is due to virus and antivirus couldnt clean it, you may need to format your drive and reinstall the OS.

Check your Ram.Take it out and reinstall it.Sometimes this will be enough.If it is damaged you need to buy new.

If it is due to fan problem the computer usually shows warning before shutting down.