backlight working for 1 sec after turning on then goes black

Hello

i was replacing the screen , and i admit i did a mistake , which is unplugging screen cable before i unplug the battery cable , even though the mobile was off ( could this be what caused the issue ) ?

when i finished and tried to turn on the phone and this problem happened

no back light but i can see apple logo barely , but when the phone is turned on , the backlight work for 1 sec and then it goes black

can i get backlight to work permanently instead of one sec ? how to do so ? is it a board problem or something installed wrong from my side ?

btw , i turned off the phone many times , and each time when i turn it on . backlight working for one sec

thanks in advance

UPDATE : i truned it off and on again , suddenly it worked ok , then i tried to raise the brightness , it went black again , but the phone is working , is it a sensor problem ?