automatic Trans, sudden loss of 3rd gear, will no longer shift into 3rd gear

1994 GMC K2500 5.7L with I believe the 4L60 trans. It is the lighter duty of
the two K2500's, so I'm going with the 4L60 until I verify for sure.

So I was driving (about 60mph) and very suddenly the truck acted as if it was
in neutral. No advance warning. I pulled over, and was able to put back in
OD, and head back on the road, as soon as it shift to 3rd, same thing. I
instantly knew I lost 3rd gear. Pulled over again, I checked transmission
fluid, found it low (Not empty) with barely anything on the dipstick, and then
found a small leak in cooling line. On the side of the road, I fixed cooling line,
refilled the trans,(it took 3.5 qrts) and upon discovering that 3rd still wont
work, I limped it home in second gear. (about 5 miles)

I did put some burn to the 3rd/4th gear when I lost fluid. You can smell it on
the dip stick. Now obviously since it was a very sudden loss of 3rd with low
fluid, something broke, plugged up or otherwise failed due to low fluid.
Before I go see about a new or rebuilding of the whole transmission, I'd like
to find what broke or stopped working preventing it from even trying to go
into 3rd. I can't very well verify the 3rd gear clutch pack is fried if it wont even
engage.

1st,2nd, and reverse all work just as strong as if nothing ever happened to
my truck. In fact, the next day, I plowed my driveway with truck in 2nd gear
and reverse with no issues.

any ideas?