On my iPhone 7 Plus, after I upgraded to iOS 12.0.01, the touch screen stopped working. I'd restart it, but the touchscreen just doesn't work at all (so I can't slide it to "off").
IOS – iPhone 7 Plus touchscreen won’t work!
iosiphonetouchscreen
Related Solutions
It is almost certain that you have a hardware problem, something wrong with the actual touchscreen. Since the iPhone 4 is less than a year old it's still under warranty, Apple will fix it for free.
If you haven't done so already you can try a complete restore of your iPhone, erasing everything and returning it to it's brand new state (after backing it up). Odds are not good that this will fix it, but it's worth a shot, especially if having Apple repair it is a hassle.
Apple explains how to back it up and restore it here:
iTunes: Backing up, updating, and restoring your iPhone, iPad, or iPod touch software
Most likely, cable nr. 2 (responsible for the touch-input) isn't connected properly. Try to re-open your iPhone, disconnect an re-connect the cable.
If that's not working, you have probably damaged the cable or something else.
It's, like Gerry said in the comment, next to impossible to determine a cause without physical access to the device.
Best Answer
Unfortunately, Apple is no longer signing iOS v11.4 so you cannot downgrade the iOS version. I've seen a few scattered instances of iPhone 7 touch not working on iOS 12. The common denominator seems to be aftermarket screens.
This is similar to the issue we had back when iOS 11 came out and lower quality aftermarket screens were also affected but on a much wider scale. You could contact Apple support or Store but they will probably just want you to replace the screen for an Apple original.
If you can't or don't want to do that, then go back to the shop that repaired your screen in the first place and see if they have an "original-quality" screen they can try to help troubleshoot the issue. Otherwise, you will have to wait until (if ?) this gets corrected in the next release.