What is up with my Sony Xperia Z5?

Hi there,

A few weeks back I dropped my Z5 and the glass screen was smashed. The LCD Display was fine but the touchscreen was not working.

I took it somewhere to get repaired and its taken them 3 weeks to try and fix it and have failed to do so.

They have fixed the glass yet whatever screen they use to replace the broken one makes the display show lines on the phone. They claim its a problem with the motherboard... but the LCD screen was FINE when I gave it to them and suddenly its gone lines on it and they can't fix it. Could the motherboard be broken?

Does anyone have any insight? I don't see how the LCD screen being broken is MY fault because it was absolutely fine when I gave it to them and now its screwed up. Is there a chance they have broken my phone further trying to fix it? Is my phone salvageable? Please help.