I have an 84 mustang with a Borg-Warner T5 (NWC) transmission.  A
couple of weeks ago while I was driving I lost 5th gear.  No noises or
crashes were heard.  Now when I shift into 5Th the car acts as if it
is in neutral.  I still drive the car daily using it as a 4 speed. 
Can you tell me specifically what may have went wrong internally with
the transmission?  I would like to have some idea what i am getting
into before i tear it apart.

I am not a transmission expert but am looking in the Manual
Transmission section of the Chilton Auto Repair Manual. They call this
5-speed a Chilton Type 13 for their book. There is a snap ring at the
end of the assembly which holds the 5th gear Syncro.Hub Plate and
Sleeve onto the shaft. Maybe this fell off and allowed these parts to
move back out of play? I'd drain the fluid and see if you have any
metal in there. Hopefully the snap ring will appear. If there is not a
magnetic drain plug there, I'd install one. You should buy a used
Chilton's Manual and see what you're getting into before tearing into
this trans. I bought my manual on eBay for $10 plus shipping.

I don't have my trusty Hayned manual with me here, but normally with
the B/W T5 tranny 3rd gear is the one to go especially if you drive
the car like you stole it. That said, these transmissions are very
simple, but shops can charge you an arm/keg to work on it regardless.
My suggestion would be to swap out the transmission for a working one.
$300 (and that's canadian dollars) can have you a new one with a
warranty from your local junkyard. Assuming this is a later model fox
body Mustang, swapping the transmission is as simple as;

-Removing your shifter from inside the car (4 bolts, and it's located
under the shifter boot)
-Dropping the h-pipe (8 nuts/bolts)
-Drop the driveshaft (4 bolts, you will need a closed ended wrench to
get this one)
-Remove cross member (2 nuts/bolts) and remove transmission mount (2
bolts)
-Remove transmission from bellhousing (4 more bolts) It pulls straight
out, and when drained of oil doesnt weight more than around 50 - 60
pounds.

The car should be raised up as high as you can safely get it to make
things easier. It helps to have another set of hands to remove the
transmission once the crossmember is unbolted.

Hopefully I havent missed anything, and this is always easier said
than done, but 8 hours should be plenty of time for the  do it
yourselfer to finish this job. Once you have the transmission out, it
would be a great time to buy yourself a stronger clutch !

Answer:

There is a small shaft running down the driver side of the tranny
(internal) that has a small pin running through it, this pin does two
things.

One: keeps the shaft from spinning (which would render it useless)
Two: allows the shaft to push or pull the assembly into 5th (also used
for reverse syncro)

This little pin had simply become non-functional in my case... this
allowed the transmission not to shift into 5th and also rendered my
reverse syncro useless.

To test to see if this is your problem is simple.  Go forward at a
slow speed (SLOW!!).  Push in clutch, place into reverse.  If it
grinds then the syncro for reverse is not working and that is most
likely your problem.  The missing pin is most likely located on the
huge magnet in the bottom of the tranny.

If not, sorry.

Reverse will grind regardless of what speed you're going.  NEVER put
your car in reverse while the car is moving forward.  Reverse is non
syncronized.  This is the reason it grinds while going from neutral to
reverse.  To stop the grinding, you (with the clutch in)engage 5th
(for proximity), a syncronized gear and then into reverse.  The reason
3rd gear is the one that usually goes is due to the shifter
configuration.  There are no gear lever stops on the shifter.  After a
few good powershifts, you could bend the shift fork, causing later
shifts under hard driving to side load the gear.  Under the stress,
the not fully engaged gear will strip.  As far as your problem, I'd
guess it could be a fork issue.  The best thing would (obviously) to
take it apart and see what happened.

 - Jeff