Ifs fine I have fixed it. the Modular PSU had a motherboard cable that had the end you plug into the MOBO while the other side had 2 headers on it, one big one small. The small one wasn't clipped in right and my dad had to come help me clip it in (my old PSU was none modular so I haven't ever used a modular one before).
Also just in case there is someone reading this who does not know how to plug the motherboard cable into the PSU you have to plug the big and small part of the ATX cable into the PSU so it will look like two cables plugged in the PSU MB slots then the cable will join into one where the over end plugs into the board.