Pc does not boot after installing new psu

Hello,
i have a custom made pc about 3yrs old
recently the psu was blown out
i've replaced it with a rocketfish 500 watt psu
it booted up nicely then freezed up
i override it then to no avail i wont boot up again can some one plz help me
specs
2gb DDR2 PC2-5300-667 DIMM memory
pentium dual-core 1.88ghz processor
160gb hard drive
P4M9OOT-M mobo
onboard graphics card
i've tried resetting the cmos
booting with from one hardware & numerous other things
the pc was not in use for over 1 yr