If you loose a gear, it means the clutches for that gear are destroyed. Can you manually put it reverse and have it go? First? Second? Third? etc. Even if it is an automatic, you can still put it in Low, 2nd and drive. What year/make is the car? Is there a possibility that the trans fluid leaked out and you drove it with no fluid? Like TK said, it could be something wrong with a computer. Newer cars have a lot more electronics and such than older one. That's why I have two older cars - an 86 toyota camry diesel and a 65 ford fairlane. =)