2001 Mercury Sable Trans Issue??

Hi. My son has a 2001 Sable Premium Edition with 77,000 original miles on it. He was at a stoplight and the car would not drive. The engine kept running but it would not drive in any gear. There was no warning that the trans was going bad...the car just stopped. No noise when shifting into any gear, other thaan a slight whirring noise, if that make sense. I checked the trans fluid and it was pink with little or no metal shavings. I am getting it towed this weekend to a shop, but wanted to get an idea if I was looking at trans itself or maybe an electrical issue?? Thanks in advance.