Make sure you’re running the latest version of iOS, you can check by going to Settings>General>Software Update and doing an update if needed. If the iOS update didn’t fix it, or if you’re already running the latest iOS you could try doing a DFU restore. This will cause you to lose all information on the phone so doing a backup prior to the restore is strongly recommended. To do a DFU restore: (1) Plug your iPhone into a computer running iTunes (2) Press and hold the Side and Volume Down buttons simultaneously for about 8 seconds (3) Let go of the Side button while still holding down the Volume Down button (4) After about 8 more seconds, iTunes will prompt you to restore the phone Hope this helps!
Try hooking up a known good battery and chargeport (as well as the screen you have), plug it in and give it a few mins to boot. Once you’ve given it enough time to boot, try plugging it into a computer to see if the computer detects it. If the phone is detected but the screen is still black it could be a faulty screen, or the phone is in DFU mode. If the phone is in DFU mode, when you open iTunes you will get a message saying “iTunes has detected an iPhone in recovery mode. You must restore this iPhone before it can be used with iTunes”. To exit DFU mode, simply hold the lock and home button down simultaneously until the phone reboots, or if you don’t mind losing the data, click restore in iTunes. If the screen still does not show anything after this, the screen is probably to blame. If your computer doesn’t detect the phone there might be bigger issues. With plugging/unplugging the device it’s possible that the charging IC was damaged, in which case it would need to be brought to a repair centre for...
This is likely caused by an issue with the audio IC chip on the board and will require microsoldering to repair. If you have a repair shop in your area that does iPhone microsoldering, bringing it there might be your best bet. Hope this helps!
Is your iPhone getting very hot? If so, there’s some sort of issue with the board that’s causing it to heat up to the point where the phone is considered unsafe to use. If it’s not heating up, just giving you an annoying notification, then there is likely some sort of issue with the temperature sensor on the board. Does the inside of the phone have any clues as to if it’s been water damaged before?
When the screen came off it is possible that something jostled around and possibly is causing a short somewhere in the phone which is resulting in the heat as well as failing to connect to a computer. Replace the screen and see if the issue persists
iPhone 8 Plus’s dual rear camera shares the work of taking pictures from near and far between 2 cameras. One camera will handle the close up shots while the other one handles anything that needs more zoom. If your camera works as soon as you zoom in to 2x it likely means that one of your cameras will need replacing. Doing a factory reset can fix some issues, and it may be worth a try before opening up the phone, but in my experience this seems like a quick camera replacement should solve this issue.