Is my cpu broken or my graphics card

[720boy]

So i had to apply new thermal paste to my cpu and as well as my graphics card (GeForce gt 720) and i put it all together and start up my pc it starts up normally accept my monitor turns on for a split second nothing pops up in that time just black and then the monitor turns of as if my pc was off it self but my pc runs the entire time. Please help me i need to know what to replace. Im honestly worried it might be my motherboard as well i didnt do anything to it that im aware of so how can i rule that out?

 

[Guest]

Lets clear some possibilities:
Do the following.
1- Unplug the GPU and then Plug your standard On board Graphics, If everything's OK then it might be the CPU or the MtB.
2-Once step 1 is cleared and it still has the same issues. Connect back again your GPU and then try accessing Set-up CMOS by pressing Del when you turn on your computer, then check in the screen the CPU and all its cores are listed.
3- Do a full reset of your motherboard settings.
4- If all else fails, Its your motherboard.