Intel Mac Pro shows blinking light on startup, nothing else

On my Intel Mac Pro (system 10.6.8; 10-12GB RAM; system on a 250GB HD, data on another 250GB HD), the startup button lights up, starts the fan (softly), then continues blinking... but no startup ding, nothing on screen. Tried zapping PRAM, starting with SHIFT key, starting with OPTION key, unplugging and plugging power cord. This happened once before, but the problem corrected itself, so I purchased a 1TB WD Passport Ultra for the Time Machine backup. But, I can't see the Passport on my Windows laptop, so can't get at my data. I also had a 500GB hard drive go out a year ago.