If you recently made your iPhone 8 with a third-party repairer of the display, then there could be a problem. If you already have iOS 11.3 installed on the device, then the repair process will not be successful. It is speculated that Apple uses a custom microchip that connects the device to the display. If a new display is now installed, it must be reconnected to the microchip. If this is not the case, all touch input from iOS will be denied and the iPhone will be almost non-functional.
While it’s still unclear whether Apple will fix the problem, there are also some stores that will not make any display repairs on the iPhone 8. Last year’s focus was on security, so third-party vendors could not just swap the Touch ID sensor. For example, this could have allowed access to the fingerprint, or it would have been possible to circumvent the lock. However, why the multi-touch display does not work is unknown.
Incidentally, the repair shops are also the iPhone X similar grief. There, for example, the ambient light sensor can not be exchanged without the Face ID being deactivated. However, safety is also the focus here.