EDIT: Disregard the following. I misread your post.
By the MOBO light, do you mean the so called "flea light"? A small LED of the motherboard itself? If so, it's sadly not a guarantee that your PSU (or motherboard for that matter) is working correctly.
I'd try reseating all the connectors to the motherboard as well as reseting the BIOS, preferably with the CMOS jumper. If that fails, perform full a teardown. That is to say, disconnect one part at a time from the motherboard and try booting without it. That way you'll find if any particular device is preventing a boot.
Good luck.