Turn off, unplug the computer.
Perform a power drain. Press and hold the power button for 30 seconds.
Now, put the OLD graphics card back in the mainboard, reconnect the power and boot.
If it boots, then you know the MB is good. (If not, the MB is fried and needs to be replaced....)
Next, if the MB has been found to be good....
turn off, unplug the computer. Always good to do another power drain for testing.
Take out the old graphics card and put the new one in.
MAKE SURE the card is properly inserted and ALL CABLES are connected before connecting the power.
When you boot, immediately get into the bios and check to make sure all settings match the NEW card.
If the computer continues to reboot, either the new graphics card is fried, or it is incompatible with your MB.