Obviously there can be many possibilities. If you didn't bend any pins when removing the CPU it should still be OK. Is there more than one 12v power connector on the GPU card? If so you might have missed connecting the auxiliary GPU 12v power? If your mobo model used LEDs for trouble shooting, you might be able to find the LED codes in the OM or online.
Your local PC shop? Just give them a bell and say you just want to find the faulty part (cheap), possibly replace (only if you want).
They will be grateful for the business.