This is a great question and allow me to answer it. in order to fix or replace the camera lens on an iPhone se you must go through the front. You will have access to the center through the back but only if you want to damage it or disable it. I recommend the iPhone SE iFixit teardown to fix your issue. iPhone SE Teardown
Now there is a lot of reasons this may have happened. So let me ask a few! (1) While repairing the device, did you have any liquids or water near it? (2) Were you wearing anti-static gloves or did you feel a shock while fidgeting around? (3) have you made sure all the connections to the motherboard are secure and placed properly? (4) Is your power strip/wall outlet dead?
While there is no power going trough the Mac, Hold the power button for 1 minute, afterwards plug the power supply into the MAC, BUT NOT THE WALL! Again hold the power button on the Mac for 30 seconds. This will drain all power in the supply
This is caused when a wire becomes loose. A display flex cable may have been unplugged To fix this, unplug the iPhone and open it up On the bottom right there should be a long metal plate with 2 screws in it Remove the screws and unplug the flex cable This is the battery Now do the same to the display flex cables. Now plug in the Display flex cables only and secure the base plate to the display flex wires After that go ahead and reconnect the battery DO NOT POWER IN THE DEVICE YET now put the plate back on the battery flex cable and carefully close the phone back up Now try to power it on TaDa! You should be powered on!