Hi. Did the service man have replace both the LCD and touch? Kindly reply below. Update: The foregoing defines your possible situation.
- The touchscreen/panel ribbon was not fitted properly. It will need to be redone
- The touchscreen got partially damaged together replaced LCD (this will depend of how the LCD came to fail).
- The LCD is a generic replacement and hence it does not offer the same brightness as the replaced OEM part (this also will depend on how bad the lack of brightness is - the issue may also be due to a defect replacement LCD
- Though highly unlikely, this may be a software issue. You could try a reset but a firmware reload will most likely be the only recourse if it is indeed a firmware borne issue.