My wife was driving our '91 LX with a 5.0 yesterday when suddenly the engine lost power and shut down.  She doesn't recall any warning lights going on.  The car cranks fine and all the accessories work.  The fuel pump seems to work but I'm not 100% sure of that.  I'm guessing the alternator, but, before I tear that out, I was wondering if anyone else had any other thoughts on wear to start.  By the way, the car has about 86,000 miles on it.  The spark plugs are original and the car is very well maintained (we always get asked at gas stations if the car is new).  

Any thoughts and or suggestions would be greatly appreciated.  Thanks!  

There are many things that can cause this but they all boil down to the basics - fuel and spark.  Take a spark plug out, reconnect the wire to it, ground the threads, crank the car and see if it sparks.  If it doesn't, at least you know you have an electrical problem.  A simple code reader for a 91 isn't very expensive (I paid $30 for one for my 91 Explorer and it would probably be the same model for a Mustang.)  Buy or borrow one and see what the engine computer tells you.  Another thing - There is a "kill" switch that trips in case of an accident that turns off the fuel pump.  The owners' manual will tell you where and how to reset it.  (I don't know where they are on the 86-93 models.)  It could be something that simple.  An alternator failure would cause a dead battery and you wouldn't be able to crank it so I doubt that is your problem.  I wish I could be more help.  I am sure others will chip in with tips.  Let us know.  

You need to access the onbaord computer and look for a code in the memory, the easiest way to tdo this is by using a code reader. Here in So Cal, Autozone has them for use for free. I'm not sure if they let them out of the parking lot though.  Alternatively you can count the flahing ceck engine light to get a code if you get the procedure from a manual. It is definitely not a bad alternator.  

Okay, after a few days of kids coming and going and getting my wife driving again, I've had a chance to look at the car.  I pulled one of the spark plugs and reconnected it to the wire.  I held the plug against the black ground clip on the front driver's side of the engine.  We cranked the engine and there is no spark.  I can smell plenty of fuel, especially with that plug out.  I'm about to try the code reader route but before I do, any thoughts on this?  I've got a '69 and '70 sitting in the garage next to this one but this 5.0 seems like a visitor from Mars compared to those 2.

Flag on the play.  Upon further review of the engine compartment, I have found what looks like the coil wire to be broken off from where ever it attaches, not on the distributor side (or what looks like the distributor).  I'm going to check some on line engine diagrams unless someone knows where that connects.  

Thanks to both of you that replied to my post.  It turned out that the coil wire burned through at the top of the coil.  It wasn't readily apparent as the coil is covered by a plastic shroud.  The suggestion to check for spark lead to my tracing the source of each wire as my plugs were getting no spark.  Again, thanks very much.  I only needed to install a set of new wires...but I put new plugs in as well.  

The kill switch is located in the hatch area behind the plastic trim.  The switch is mounted to the tail light panel.