If the Coputer doesn't boot there are several possibilities which could cause that
1) Your mainboard need a -5 volt signal
This currency isn't part of the ATX-standard since ATX 2.0. Some mainboards, specially older models still need it to boot up.
In this case please contact your dealer.
2) The PSU is dead
Your dealer can replace defective PSUs unbureaucratic and fast. Some online-dealers might ask you to send in the whole PC-case, others just want you to send in the PSU.