I have a '99 Town and Country. I had a problem with the belt tensioner working correctly, such that my belt would squeal sometimes and one time did come off completely. As I recall, I had the belt tensioner replaced a couple of times til the problem as fixed (~50K miles ago).
Mine came off twice. The car is a 2000. A friend said to check the belt to see if it has stretched. Otherwise, I cannot figure it out.
You should probably check your tensioner pulley. Go to autozone.com and sign up. After that you can access car repair instructions.