Please diagnose before doing any repair.

It seems wise to check for pending diagnostic trouble codes.

You can check the fluid by looking at it on the dipstick and spreading
a few drops on a white paper towel.  If it is transparent, is not dark
and does not smell burnt, I am prone to believe the fluid is not the
culprit.

Once I thought I had a serious transmission problem: it was nothing
more than a worn rubber bushing that links the wire that comes from
the shift stick to the transmission lever!  It does not seem to be
your case.

Good luck.

Hello,

     All of a sudden this morning my sister's 99 Elantra transmission
will not shift into reverse. Also the transmission will not shift out
of first gear. Last night when she came home everything was fine with
the car. The check engine light has not come on yet, but I will try to
read the codes off and see if there are any codes that are stored in
the ECU. I will also drain and fill the transmission fluid and see if
that helps, but it wasn't due for the 30k fluid change for another 10k
miles. Does it appear that the transmission is shot? I was also
reading that it could be something to do with the vehicle speed senor,
but wouldn't that throw the check engine light?

Thanks,
Nick