why wont my car shift out of third gear? on 2001 Volkswagen Jetta

it shifts all the way til third gear, but wont come go into overdrive

Check the transmission fluid level, if it's OK then I would have a diagnostic check performed on the transmission. There are so many possibilities, and only an inspection will determine what is going on here. Go to a shop that specializes in VW and/or transmission repair.
1 more answer
hows your shifter alignment? does r,n,d line up? the linkage could be off, not usually likely unless some work was done in this area. does the shift display light up when this happens? if so it stores a fault and can be diagnosed by a tester. should check for faults anyway.