Do the fans spin up, any other LEDs etc?
If no fans then either the PSU is busted, the fuse is blown in the kettle lead, the switch isnt on at the back of the PSU, the case switch isn't wired to the board's header correctly, the case switch is broken full stop (in which case jumper and un-jumper the header on the board to see if it boots), if all of that fails then try to jump start the PSU by connecting which ever two pins it is (I dunno).
If the PSU doesn't even jump start by jumping its own pins then assuming the fuses are ok and the psu is on, then the PSU is the problem. If it DOES turn on, but jumping the board doesn't work, then you have a faulty board.
P.S You did remember to connect the 12v CPU aux and all graphics card connectors right?