Welcome to the club, like mesab66 I think everyone single one of us had one of those "Ahh $@*^" moments like this. So here is some short but helpful advice I learned over the years.
If the problem seems complicated the answer is simple. Another words check for the stupid stuff like cables connected, etc even if your 110% it's right.
If you can't figure out what the problem is, find out what it's not. So lets say it's trouble getting a system to turn on, rip out every part except 1 stick of ram and cpu and slowly start adding things so you can eliminate what it's not.
Don't let your family or friends know your a computer geek unless they are too. You will never get rid of them as you are now free tech support. Sounds jaded but after spending over an hour of telling someone how to Copy & Paste and various calls like that you get here too.
If you do end up being tech support and building or setting up said PC for family. Follow one simple approach make it stupid proof as much as possible. Automate virus scans, defrags, updates, etc so they mess with it as little as possible. Which means less chance of them braking it.
Oh strange advice on this one but I am sure most of us did this at some point (though I probably had it worse than most). Don't buy cheap cases it's easy to cut yourself on it. I bled all over my motherboard and am surprised I didn't need stitches, and I swear they used some type of chemical bath on that case and didn't clean it properly or something as it burned. And cleaning that motherboard/GPU/etc is not fun, plus you can fry parts that way. So pay the extra $10 on something with rounded edges and good build quality.