Hyundai Repair: 2001 Hyundai Sonata, hyundai sonata, serpentine belts

Question
My Sonata will go up to about 20 mph, the rpms go up to about 4000, then the transmission feels like it  disengages. It doesn't feel like it ever shifts out of 1st gear (it is an automatic), but no lights have come on. We recently had the timing and serpentine belts changed, and have driven it a couple thousand miles since. The top speed it will go is 20 mph. What should we be looking at or for?

Answer
If it feels like it just shifts into neutral, I'd recommend checking the fluid level.  If okay, you likely need to replace or overhaul the transmission.

If it just stays in first gear, then you might have a problem with the shift cable or range switch (or the adjustment of either).  Once again, if okay, expect to need to replace or overhaul the transmission.

If it looks to you like it's going to need transmission replacement or overhaul, I recommend taking it to the dealer for actual diagnosis.  This is too expensive a repair to be guessing based on the information I can glean here.  I do not recommend taking it to a transmission chain-- most seem focused only on transmission overhaul.