Q: Car lost ability to accelerate going down the road, then eventually died.

If there were other bad behaviors leading up this failure, like backfiring or surging, I suggest looking for any apparent air leaks in hoses leading into the engine. Any air that gets into the engine without passing through the Mass Air Flow meter will cause the car to lose power and even shut off. If this happened suddenly without any other events, I would guess that you have a problem in the throttle motor. In any case, there should one or more trouble codes stored in the engine controller to indicate this or most other failures that would lead the car to shut off on the road. The next step in a proper diagnosis is to plug in a scanner and read the codes. You can have that done at your convenience by contacting Your Mechanic. They can send a technician to your home or office to check out your Nissan and let you know what it will take to fix it.