What did it turn out to be? Just happened to mine yesterday. Tried replacing the battery (which did nothing) so I had it tried to have it towed...but then it wouldn't come out of park. Mechanic had to come roadside and managed to bypass a lock-out fuse and it was towed. Now waiting to figure out what it might be.
What did it turn out to be? This just happened to me as well, but I forced it out of park and broke the bushing on the cable to the transmission, after that now It will not give me back the Ignition Key?