Computer went off on reboot and won't come back on. What else can it be besides power supply?

I was rebooting the computer when it just died. The switch will not do anything. I tried a new power cord, nothing. I have a power supply tester and it says that the power supply is good. I checked the hard drive and it seems to be fine. Can RAM just go bad and cause this to happen? All parts are less than a year old.