I have a Mac Pro early 2008. I'm sending this message on my iPhone 4S. Comcast cable had an outage during a storm in our area. I was using my Mac Pro when service went to the dogs. After several false starts, Comcast finally came back. Two other Macs in our house were back in business but my Mac Pro showed a flashing white light above the On/Off button instead of the normal steady light. I am able to shut the off the flashing light by holding the button down for a few seconds. This morning, the situation has not changed. I do have an external backup drive that backs up at least every hour. I'm not worried about data loss, but I am very interested in how to get the Mac Pro back in service.
Mac Pro dual Quad-Core Intel Xeons Early 2008; 16GB RAM; MacOS X 10.11.6, iOS 9.3.5