My 2001 Ford Windstar with 100,000, was driving fine as I stopped at a traffic light. When light turned green, I put my foot on the gas, and the car would not move in drive or reverse. The engine never stopped running. Mechanic says it needs a whole new transmission. Have you heard of any other types of causes for this sudden failure i.e.. Computer-related problems?

Been there, done that, and it is very likely that the root cause is internal transmission. Sorry to give you the bad news but most of the scenarios that you are describing are internal transmission. But without diagnosing it, I'm just taking a guess based on experience. Here's to hoping I'm wrong.