My PC is shutting down, is it because of overheating?

[Charlise]

Specs:

INTEL CORE i3-4170
ASROCK H97M ANNIVERSARY
GSKILL RIPJAWS X 2×4GB DDR3 1600MHz
PALIT GTX 750 Ti DUAL OC 2GB GDDR5 128BIT
KINGSTON 120GB SSD
WESTERN 1TB BLUE 7200RPM
CORSAIR VS450

So my pc is, 2 years old, and it didn't happened before, I am suspecting that it is shutting down because I haven't been able to change the thermal paste that was pre applied into the CPU and the GPU before I bought it. Also, I am a noob when it comes to detecting the cause of the problem, but whenever I play games that require a bit of extra power like GTA V, NBA 2K17, and some other games that require a GPU, it automatically shuts down without any notice after a few minutes of play. Also, I decided to remove the GPU if the problem still occurs, but to my surprise, after using the computer without the GPU for almost 2 hours and gaming on light games that doesn't require the use of a vc, it didn't even shut down. So I am very much confused on what may be the problem, is it overheating or my GPU is dying (I hope not)? Thanks for your answers.

 

[Sedivy]

Get something like HWInfo or HWMonitor and check where you temperatures are at.

 

[Charlise]

Now I tried inserting my GPU again, it shows a display that a pc that doesn't have a GPU shows. Also, I can't look at the settings of the NVIDIA control panel when I try to right click and refresh. I think the GPU is not working?

 

[Sedivy]

It's likely just not seated properly. You're going to have to take it out and try putting it in again. Carefully press until it's fully seated, put in the screw affixing it to the chassis and try again.
If still not working, try booting in safe mode and see if you can detect the card at all.