iPhone 6s freezes, no screen or button responses

A few months ago, I had my cracked screen replaced on my iphone 6s and since then, at least a few times a day the screen freezes and becomes unresponsive. That would be fine if I could power it down and restart, but the home/power buttons also become completely unresponsive. I have to wait for the phone's power-saving to go to lock screen after a few minutes. As soon as I've done that, I can power back up, get in fine, and use my phone as normal.

This usually occurs after I've put in my thumbprint or passcode and just got onto the home screen or whatever screen I was on before.

I would go back to the store I got my screen replaced, but I've lost the receipt and I don't want to pay a whole load of money again to get the screen replaced a second time... hoping someone has some ideas as to what's going on...