Welcome to AF taliawhat
Nieltab might have a point there about it being hardware related, but you had mentioned this happens when you run an app?
Try this, pull the battery....wait about a minute, then reinsert battery.
While holding the volume down button, press and hold the power button.
The next screen that should come up is your bootloader screen. If that does not come on, then you for sure have a hardware issue. If that screen comes up, first take note of what it says and post that info back here.
In that screen, navigate the options with the volume up/down buttons and use the power button to make the selection.
Also from that screen, you can choose to reboot. If you dont see that option, highlight fastboot and select it, then you will see the option to reboot.
There is also an option to clear storage, but that will wipe your data, which you said you don't want to do.
See if you can reboot from that screen.