Question pc is not booting up when plug in gpu

[QuadCore Gamer]

My pc is not booting up, what i found is when i plug the cable in the onboard gpu everything working fine but when i plug the cable to gpu pc is not booting up. How can i fix this problem? this problem happens first time to me , before everything is working.
specs
dell optiplex 990
i5 2400
12 gb ram
asus gtx 750 1gb (runs without power cable )
PSU 300 watt max

is my gpu is near to dead? it is only 3 years old

 

[Fix_that_Glitch]

What does it say, if anything, next to gtx 750 in device manager?