2000 plymouth voyager transmission problem

I have a 2000 plymouth voyager that was working fine for a while and now the transmission won't shift gears please tell me what could be wrong with it.

When you say it won't shift gears does that mean you can't move it or rolling down the road it will hang in this gear or that? If it won't shift higher than second gear then it's in what is called "limp mode". This happens when the computer (TCM) detects a fault then limits shifting and speed to save the transmission. Is the SES light on?