It does not turn on.

[afefcfe432dr43]

One day, the B&O all-in-one computer didn't turn on. No led light, no sound when I pushed the power button.

I opened the case because I tried to take an HDD.

After I pull the HDD out from the mainboard, I connected power cable and tried to turn it on. It worked!

I assembled all and I used the computer for 3 weeks.

Today, the computer showed the same symptom. 

I opened the case and pull out the HDD and RAM and put it in again. But it didn't work.

I don't know what is the problem. I think that the motherboard has a problem.

I bought it 3 years ago so the warranty is expired.

[Lil_Boy_Blue]

Hi
First:- Turn off the computer, press and hold the PWR button for 4 seconds.

Then - Leave only keyboard and mouse connected, disconnect all externally connected
devices such as Docking Station, USB storage devices, displays, and printers.
Unplug the AC adapter from the computer.

Next - Press and Hold the Power button for 15 seconds to Hard Reset.
Rinse and Repeat; Re-Connect the AC adapter.

NOW begin checking/testing/proving the Hardware is fit for purpose.
Power on your PC, press ESC (repeatedly), then F2 at the prompt.
Choose System Tests (Fast Test does a 4 minute hardware check).
AND/OR
HOW DO I USE DIAGNOSTICS?

HP PC Hardware Diagnostics Windows is a Windows-based utility that allows you to run diagnostic tests to determine if the computer hardware is functioning properly. The tool runs within the Windows operating system in order to diagnose hardware failures. HP PC Hardware Diagnostics Windows comes pre-installed on new HP computers, but if not on yours, download here:-
https://www8.hp.com/us/en/campaigns/hpsupportassistant/pc-diags.html