It can be many thinks:
TFT ribbon cable moved (really hard to see by eye): disconnect and reconnect it.
Wrong soldering, 5v or USB+ short with something.
Wrong connection on compute module or sdcard : disconnect, blow air and reconnect it.
Does it work back if you unplug addon board?
Problems occurs right after 'physical' modification or software modification?
In my setup, if addon board is not plug all the way, I have some I2C troubles, sometime not able to power the system.
TFT ribbon cable moved (really hard to see by eye): disconnect and reconnect it.
Wrong soldering, 5v or USB+ short with something.
Wrong connection on compute module or sdcard : disconnect, blow air and reconnect it.
Does it work back if you unplug addon board?
Problems occurs right after 'physical' modification or software modification?
In my setup, if addon board is not plug all the way, I have some I2C troubles, sometime not able to power the system.