if it was the PSU nothing would power up
are your fans connected to the PSU then yeah your fans will turn on, but if you have your fans are PLUGGED into the MOBO and the fans are spinning
your ram slots on your board could of went out..... or your CPU went out
and if you have multiple sticks of ram plugged into your board try just removing one and try to boot
try diffrent ram to see if theres a problem with your ram
try a diffrent MOBO
and see what happens that will definetly narrow down whats causeing the problem