Have you migrated the Front Camera & Sensor flex, as well as the home button to the replacement screen before booting up the device? The iPhone 7 is finicky and requires everything to be present.
If that is already done, then put back the old screen to see if the phone boots properly. If it does, then there may be something wrong with your replacement screen. If both screens no longer work, then you may have damaged something on your logic board. Check the connectors to see if they are in good condition.