how do you know that everything is ok if the system won't even power on?
since you really didn't give much detail, i'm going to give a basic troubleshooting solution. the fact that you can't even power the system on is a big deal....
make sure the case power switch is connected correctly to the mb, and that ALL JUMPERS ARE IN THE CORRECT DEFAULT POSITION.
make sure the power supply is properly plugged into the board and switched on (the main switch on the back of the PSU).
if there's a power light on the mb, make sure that is lit.
if you can use the power switch to power it on after that, it might be a problem with the PSU, and/or the power switch on the case itself.
anything further than this can't be diagnosed because you'd need power to see if anything else is wrong.