2001 mitsibishi Galant trans failure

My wife was driving home , and the car just all of the sudden acted as though she had put it into neutral . It reved way high . There were no gears at all . She coasted off the freeway , and stooped . The car would not move in reverse or any drive gears . I checked the fluid , and it was a brown color , and it appears to be over filled . We bought the car new , and haven't put any fluid in it , nor changed it . The car has 65,000 miles on it . It is a 2001 mitsibishi galant with the 6cyl . engine . I'm figuring that I'm looking at a large expense , however I was wondering if anyone has experienced a similar incident , or knows what the problem (s) might be , or of a possible simpler solution ? I would appreciate any related feedback . Thanks ..........joe