Why won't my black screen wake up from sleep?

I can now only wake it up by plugging it in to charge from a backup battery or wall plug. When I do that the phone works fine.

It used to wake it up using the volume on/ off/ vibrate button. This stopped working when I changed the alarm sound my wake up alarm in the phone alarm.

I am running IOPS 7.1.2. The Home and on/ off buttons no longer work so I can not hold them both down simultaneously. I use the ‘assistive touch’ workaround to get to the different functions.

It used to wake it up using the volume on/ off/ vibrate button. This stopped working when I changed the alarm sound my wake up alarm in the phone alarm.

I have also tried a factory reset but it still doesn’t wake.

Many thanks.