Sounds like a defective PSU. Try disconnecting the power cord. Then press the power button to discharge any residual power in the system. Then reconnect the power cord and press the power button. If the PSU starts the system, then it is failing and needs replacement.
BTW, do you have another PSU that you could swap in to your rig to isolate the potential PSU problem?
EDIT: I have to start editing my posts before I submit them. Sorry.
Corsair CX500 seems to have died after only 9 months of use, originally my GTX500ti would keep crashing, often requiring a reboot, then after a powercut, only the dvd drive and PSU fan would spin up.
Paper clip test, no power to any component, but PSU fan spins?
Does this indicate a dead PSU? thanks
Sounds like Corsair needs to replace their quality control staff, I just RMA'd a Brand new(less than 15 hours on it) TX-650 PSU because the fan wasn't working and it was shutting my PC down due to overheating problems. Corsair also doesn't give a rat's behind how old the PSU is either it's on you and your dime to send it back and pay for the shipping, cost me almost 20 bucks to send mine back, you are also looking at, at least a 3 week wait for the replacement PSU if you live on the east coast to. Last Corsair product I'll ever buy!