1) incompatible CPU without a BIOS upgrade
2) RAM installed in the wrong slots or are not properly pushed in
3) non-compatible RAM
4) did not install the motherboard standoffs
5) did not plug in the CPU 4-pin ATX/8-pin EPS power.
6) plugged the HDMI/DVI/DisplayPort into the motherboard port despite having a GPU or vice versa
7) faulty front panel switch/wiring