Check the fluid to be sure it's full. If not, take it back to have it
corrected-- they failed to fill it correctly or it's leaking. If it is
full, then you probably have a serious problem. The good news is that, as
transmission replacements go, the 1999 Elantra is relatively inexpensive --
I'd expect $1500 to $2200 depending on location and how much they'd like to
hammer you. The factory list on Hyundai remans has been $1508 for years,
but the dealer cost is so low, most sell the trans for around $1000.