The problem may occur due to
1. The temperature setting of CPU has been changed from motherboard setting. It may be goes to lower than 75 or 60 degree centigrade-so that the operating system shutdown the system automatically.
2. It may be possible that your CPU fan is not working and CPU is going to overheated that is why OS may shut down the system.
Below is the Solution
1. Change the motherboard setting DEFAULT or AUTO. Do it from BOIS setting during starting of system.
2. Check your CPU fan is working or not , if not check the CPU fan wire is set on motherboard or not.
3. If fan is working and you have changed the motherboard setting to DEFAULT and still you experienced an same issue please remove the motherboard battery for 2-5 second and then set the battery, may be problem be solved.
Please note Over heating of CPU may damage your CPU and Motherboard