iPhone 6 Plus not turning on after screen replacement

After completing the screen replacement for my iPhone 6 Plus it won’t power on; it also isn’t recognised by iTunes. At first it powered up completely fine, then after switching it off and screwing everything back into place the phone will not power back on - although did keep track of screws so not sure it’s long screw damage. The only thing of note is that the 1.5 mm screw on the display bracket wouldn’t go back in. Are there any trouble shooting steps to try to find out what the problem is?