Why does my phone keep on shutting down every time my screen sleeps?

I've had my phone for two years now and never ran into a problem until now. My phone would always shut down when the screen times out or when I press the power button to make it sleep. I initially thought that the problem was my battery so I charged it normally but when I tried turning it on, it wouldn't.

I charged again for another hour but still could not get it to work. After the phone shut down, I try to turn it on normally using the power key but the phone just vibrates with no display.

I finally got to turn it on when when I tried soft reset. It takes at least two minutes and several attempts of holding down the volume key and power key before the phone will turn on. When it does, it functions normally and I see that the battery is fully charged. And then the phone goes to sleep and shuts down again.

I have tried the following to no avail:

1. Uninstall all my apps
2. Soft reset (offers temporary solution)
3. Factory data reset

I can't do a hard reset because my phone does not respond tho the power button - volume up combination.

Is there anything I could do before resorting to the service center? (We are currently on lock down so I cannot leave my house and I desperately need my phone for school purposes.)

Any insight would be much appreciated. Have a nice day.