Computer suddenly not posting, despite everything being on

Its very confusing, we took out the graphics card for a while to find its model number and then placed it back in the pc, when it was returned the gpu light flashed yellow on the motherboard, everything turns on just fine but it doesnt post, we have tried resetting bios, taking the battery out back in again, making sure its not the ram sticks, ensuring the strength of cables, unplugging usbs, so far nothing has worked, i am begging for help!