Solar panel problem

I am interested on solar power and am thinking of installing a solar power set up at my parent's home. My friend is installing a set up at his house, I went to see how they do it. They have four solar panels and they met this problem where when the two panels are wired for 24 volts there is out put however when the two 24 volts series is wired in parallel for a 48 volt system there is zero output. I don't think they've solved the problem. I am not an engineer and has a very basic understanding about electricity, but I'd really like to understand what went wrong because my Dad is keen on installing a solar power system so I need to better understanding of it. Thanks to anyone who can help!

The panels you mentioned were connected improperly. To add voltages of panels, you must add them in series. That means you tie one + to a - and then use the other 2 outputs.

12 + 12 = 24V correct in series but if you want 48V you need to put them all in series. You are writing you have 2 in series and 2 in parallel which can only make 24V.
For parallel wiring you connect all + poles with the other + poles (and same for - )
For series wiring you connect of one panel + to - of the next panel and so forth.

If one element in the series is dead then you get 0V on output. Are you using new panels with proper connectors?