Need help | gpu problems

[N0NAM3]

Hey guys, I'm having problems with my gpu,  750ti (gigabyte sff) and I bought a new case to move my pc with a new psu. Turns out my mobo didn't fit (it's a weird dell shape ._.) so I put my pc back together. to my dismay, my gpu did not work e=when it had for 4 months prior; I had not damaged anything in the pc or gpu and I have confirmed the hdmi cable and pci slot work by using an old gt-730 to test and some old radeons, basicaly I just took it back to the seller (ccl) and said it would power up (it did) but wouldn't show and signs of display, they tested it on their test beds for around 3-4 days, it worked, all displays working. I took it back today and same old, not working. I have since contacted the seller with proof. I think it may be the power supply as it is only 250 watts  but has a 80+ gold efficiency rating  so I doubt it would be. I am realy confused at this point and any help is greatly appreciated, thanks