screen black and no sound but itunes recognises the device

but if it was the screen surely it would ring and vibrate ?

When you say PC recognizes the phone does it say locked with a passcode, locked device, recovery mode or something else?

You did say in your post that the iPhone would ring.

If switching the phone to silent / vibrate does nothing then your phone may have a bigger problem than just the screen. There is a chance that the battery may have gone bad.

This exact thing just happened to my wife's iPhone 6. My son dropped it and when I picked it up it had lines, but was still usable. I did a hard restart of the device and it never came back on. The screen does not light up, nor is there sound and vibrating (when plugged in, while receiving calls, etc.). I plugged it into my MacBook and it immediately recognized it, but wouldn't let me do anything because there was a passcode on the device. I forced the iPhone to let me do a restore and it successfully went through everything, but the problem persists... What gives?!

Edit: I also took apart the phone and checked that the home button, display data, and digitizer cable were all connected. I could not check the battery cable because my pentalobe bit was just spinning on the one screw and I didn't want to strip it. I ordered a new screwdriver to try again on Friday.

Again. Probably the screen needing to be replaced.