Do you have another GPU you can test it with? Also, make sure you have the PCI-E connector plugged in correctly into the card.
If you do test it with another GPU and it still does the same thing, it could be the motherboard that is defective.
Sometimes, clearing the CMOS in the BIOS helps, you can do this by unplugging the computer first, touch chassis to get rid of any static electricity, then take out the button cell battery and wait a few soconds, then replace it to clear the CMOS.
If the other GPU does work and still have no luck with the 7750, then it may need to be replaced/RMA.