I have a Toyota Camry 2005, the car was taken to a mechanic for oil change out and according to the mechanic he opened the transmission down plate and remove the valve body and serviced it which I didn't ask him to. After reinstalling the valve body and the plate the gears are no longer engaging and when it engages it won't move from first to second I believe the mechanic has damage my transmission or what might be the actual problem. Any help.
The responsibilities for this rest squarely on the shop that did the work. If it was working when you took it to them, you didn't ask for the transmission service and they did it anyway, and immediately following the transmission does not work correctly.
Along with that removing the valve body is not part of a normal transmission service so I am not sure why they would have done that in the first place.