If any gpu is powered without plugging in any power cables a message is displayed on the monitor to plugin the power cables.
Dismantle everything. Remove the cmos battery and dont insert it back, short cmos jumper too. And just try to power-on with a single ram, mobo,proccy and your gpu in either slot and check.
Also, try a spare monitor or if your monitor had dual hdmi or dvi ports try those too along with various hdmi/dvi ports on back of your gpu.
