How to fix iPhone 8/8 Plus screen black issue?

Hi, This is probably too late but I really hope this can help you or others.
Firstly, we need to know what cause the Red Screen Issue:
A. Your iPhone Device Has A Hardware Problem
B. Your iPhone device has been jailbroken
C. Your iPhone device has a failed software update
Secondly, Here are some common solutions to fix Red Screen of ***** Issue:
A. Perform A Soft Restart on Your iPhone Device
B. Perform A Force Restart on Your iPhone Device
C. Restore Your iPhone's Default Factory Settings
D. Bring Your iPhone into DFU Mode
E. Update Your iPhone to The Latest Software
Thirdly, If the common solutions could not solve your problem. You can try an data recovery software - FoneDog toolkit.
Step 1: Download, install and launch FoneDog Toolkit.
Step 2: Proceed to System Recovery.
Step 3: Review Reminders before The Recovery Process.
Step 4: Download The Firmware.
Step 5: Downloading and Repairing
Step 6: Repair is Complete
More Information you can see: Ways to Fix Red Screen Issue in iPhone Device
The red screen issue in iPhone devices is truly a bothering issue to have since it can stop you from being productive in using the phone or making it fully functional. Through the common solutions we have shared above for fixing such issue, as well as the help of FoneDog toolkit, we hope that the red screen issue in your iPhone has now been fixed.
Hope it can help you!
iOS System Recovery Guide How to Fix with Various iOS System Issues

As you probably already know, there are almost an endless number of ways that your iPhone can come into contact with moisture. For example, recently, I had my iPhone 4 in my back pocket when it accidentally slipped out when I had my pants down using the toilet. Don't ask ! There is some good news however, and that is in most cases, a wet iPhone repair can be made without your having to buy a new one. Yes, dropped iphone in toilet is very, very common.

The terrific news is that all models of iPhones can usually be repaired after sustaining water damage, but this must be evaluated on a case by case basis (in my case, my iPhone could be repaired for a nominal charge).

Fortunately, iPhones consist of parts that can be readily swapped out for good ones. If, however, your iPhone has been fully submerged in water or other wet substances, chances are that the logic board in your iPhone has been exposed to moisture and corrosion has begun to take place.

If the logic board is damaged, the cost of the repair can get quite pricey, especially if other parts also need to be replaced on your iPhone. The logic board is the main circuit board in the iPhone that runs the operating system and stores all data on the iPhone. The logic board is not typically too expensive to replace on its own, however, coupled with other parts that might also need to be replaced, the cost for repair may run you a pretty penny..

Reasons for Black Screen of ***** and How to Fix it
Hi, This is probably too late but I really hope this can help you or others.
Firstly, we need to know what cause the Red Screen Issue:
A. Your iPhone Device Has A Hardware Problem
B. Your iPhone device has been jailbroken
C. Your iPhone device has a failed software update
Secondly, Here are some common solutions to fix Red Screen of ***** Issue:
A. Perform A Soft Restart on Your iPhone Device
B. Perform A Force Restart on Your iPhone Device
C. Restore Your iPhone's Default Factory Settings
D. Bring Your iPhone into DFU Mode
E. Update Your iPhone to The Latest Software
Thirdly, If the common solutions could not solve your problem. You can try an data recovery software - FoneDog toolkit.
Step 1: Download, install and launch FoneDog Toolkit.
Step 2: Proceed to System Recovery.
Step 3: Review Reminders before The Recovery Process.
Step 4: Download The Firmware.
Step 5: Downloading and Repairing
Step 6: Repair is Complete
More Information you can see: Ways to Fix Red Screen Issue in iPhone Device
The red screen issue in iPhone devices is truly a bothering issue to have since it can stop you from being productive in using the phone or making it fully functional. Through the common solutions we have shared above for fixing such issue, as well as the help of FoneDog toolkit, we hope that the red screen issue in your iPhone has now been fixed.
Hope it can help you!

Connect it to iTunes, you might need to activate it, or restore it.
that's all that means. There's no verifiable OS active on the phone