Dead screen after repair

Hi I replaced charging flex cable and went to connected my screen to make a call to ensure that the mic was working (it was working). I honestly can't remember if I turned the phone off before I disconnected the screen. Took screen off to place all the screws back. All screw were put back in correct location. Then connected screen and turned it on, Apple icon came up and then disappear. Then Back light was lit for a little bit but then it went dead. No light coming on at all. Phone was previous water damage and left some smudge marks on screen, but everything except, mic( which was fixed with replacement) & home button didn't work (ID touch worked). Is that a backlight issue or lcd? Plugged phone with cable, just keep hearing a dong noise every 2 seconds. What am I supposed to do? Do I replace the whole front panel or just the backlight?