Was the CPU fan spinning? Any LED on mobo lit ot blinking? Did you use the brass standoffs? Make sure all the connectors are well secure.
remove all the parts from the case. Put mobo on a deskp, mount CPU, heatsink/fan, memory, graphics card. See if it POST. Do not forget to make the connection of PWR jumpers. If still no POST, probably bad mobo, memory, or CPU.
If possible, attach a picture.