My 2006 Jaguar S type won't go when Drive

My 2006 jaguar s type never had  problems
three days ago went to get lunch and my car
wouldn't go on drive, you can reverse it  but
won't go on drive. You know when you shift
the shifter on drive usually it should stay put
and look on Drive right, but it don't u can put
the car on drive but won't see it click and
stay on drive . I was thinking that it was
transmission so I wanted to make sure. so
what I did is  Took apart the shifter so then I
seen a  white box once I took the gear box apart push the little white box down on drive
, then my car was able to go on drive, drove
it home perfectly but not the same strain
normal then  a light came on on my
dashboard that says gearbox fail. So when I
come home put the gearbox back how it
was now I cannot get it out of park. Is this
my transmission is going bad or the gearbox
going bad?????