yeah...what you could do is take the motherboard out and set it on the table and try to boot it then....that is standard procedure after you have already tried reseating everything.
Lots of times you find it works out of the case....so maybe it was shorting on some metal or you had a wire pinched or something.
you dont really even need the wires from the case switches etc hooked up..you can just take an old hard drive jumper and short the power switch posts together on the board to boot it..or just use a flat blade screwdriver to short them together to boot...just be a lil careful with that method.