Power not supplied

I guess it is back to the 533MHZ CPU 256MEG ram,Via Chipset,dual cpu support,Tyan S1854 Trinity 400 motherboard.It has a PPGA-370 Intel Celeron processor,ATX form factor and supports 8.4 gig and higher. The power supply was a power switch on front and on off switch in back it’s gone,was 300 watts. I purchased an ATX power supply,inserted it, turned it on and no power.I purchased it from where I purchased the computer.They know me and my machine.I returned twice because of no power.There is no back switch.All pins are on the way they were before.