Many cases will come with up to 10 standoffs installed, when the motherboard only has 5,6 or 8 mounts. Carefully line up the holes in the motherboard with the case, and remove any extra standoffs. An extra one may be touching somehting and shorting.
Some times a defective case USB plug or power, or reset button will prevent the board from turning on. After you check and remove extra standoffs, try to power up with out any of the case connectors. Power up the board with the button on it, or by touching the power button pins with a screw driver.