Try Holding L, R, A, and Up (on the directional pad, not the circle pad), then performing a system update. This fixed a “soft-bricked” 3DS. If this does not work, you may have to insert the SD card into a computer and perform a specific procedure for “hard-bricked” 3DS consoles. I found this on YouTube long ago. If you wish, you may click here for a video clip I have found, but not yet reviewed due to it being blocked.
I think that you either have not completely plugged the ribbon cable in securely enough, or you may have accidentally damaged a part. You may need to check for broken parts and get replacements for any that have been damaged or broken.