Could be a dead short or missing power cables. On the case did you use the eight brass colored screws to keep the mb from shorting out to the mb tray?
Check that the mb back plate fingers not shorting out the USB or the bottom of the mb. If you used an after market CPU cooler check that the back plate not shorting out the mb. On the top of the mb check that the four or eight pin CPU power plug near the CPU or on the edge of the mb is connected. Check that there no bent pins on the drives..that the ram is in right. Look to see that the USB cables are on the right ports and pins.
Pull the case fans to rule out a bad case fan.
Dead shot? Power a cables all connect 25/8/6 all connected. Motherboard screwed in with stand offs. Mb backplate? CPU cooler has no backplate noctua nhl9i 8 pin connected. Bent pins on ram? USB 2/3 plugged in fine and fans spin fine .