Hi
If it is a Hardware failure then simple software solutions wont help, however....
Troubleshooting is here...
https://support.hp.com/us-en/product/HP-Pro-All-in-One-3520-PC/5299218
Drivers here...
https://support.hp.com/gb-en/drivers/selfservice/hp-pro-all-in-one-3520-pc/5299218
Service guide...
http://h10032.www1.hp.com/ctg/Manual/c03577915
Hard Reset...
Press and Hold the PWR off button for 4 seconds to force a PWR off.
Unplug from the wall.
Press and hold the power button for 15 seconds to complete the reset.
OR
Power On : Press ESC key and then F2 Diagnostics.
System Restore: Power On Repeatedly press F11.
If in doubt please ask.