If you did not press the yellow button next to the battery before removing the trackpad cable, and you didn't disconnect the battery in some way, there is a high chance you have damaged the logic board.
Have had a few I've received from other shops who have done the same thing, but haven't managed to figure it out yet. On all of them however, none of the S4 rails start due to a missing PM_SLP signal required to activate them, which comes from the CPU.
Basically, a new logic board will fix the issue. Not exactly sure what on the board is the issue, but it has happened due to the battery not being disconnected and the G3H rails not being de-energized before doing anything.