It’s probably a simple fix, but if you don’t know how to open the case, I’m really worried you don’t know what you’ll be looking at once you are inside. It is unlikely the issue will be visible. You won't see a burned out part to replace or broken wire. More likely a voltage regulator or other little component has opened up and will require a technician who knows what to measure and where.
If it’s like my Luxman integrated then the bottom panel is the first thing you should remove, and then the side rails, but really if you aren’t sure give this to a repair person. For them it’s probably a quick fix.
