Faulty phone since screen repair

Hi since getting my screen replaced my phone is constantly switching itself off, comes on with the apple logo then immediately switches back on and does this constantly until I can manage to force it to turn off, it for a rare chance it does come on and let's me enter the passcode, it then freezes and switches off again, doesn't matter if I put it on charge or not, I've taken it back to the shop and they said it's not there problem as they only replaced the screen but the issue only happened since the repair???