There are many stories I could tell with this title. It's such an obvious rule when dealing with electrical devices but so often overlooked as a possibility of the source of a problem when everything else works. Here's my most recent experience that took its toll on more than just myself.
I work from home and my office is in the back of the house. Several years ago, I strung network cable from the router in my office, up through the attic, and down into the coat closet in the living room in the front of the house where I hooked up a Wi-Fi router. So we had two Wi-Fi points, my main router in the back of the house, and the secondary one in the front of the house.
Fast forward several years where Wi-Fi technology is more stable, prices have dropped, and I finally upgraded to a wireless mesh system in our house so that we'd have just one W-Fi access point throughout the house. Because of the way things were set up, I decided to use the cable in the living room as the starting point for the house-wide Wi-Fi network with a couple of repeaters spaced towards the back of the house. This worked great, even for the Roku stick on the back of the TV in the family room (near the back of the house) for streaming movies.
This year, as usual, I put up Christmas lights in early December. Living in Western Oregon, we get quite a bit of rain this time of year so I put electrical tape over the connections to prevent electrical problems. I don't always cover every connection adequately and sometimes water is detected in the line. When that happens, the GFCI outlet disconnects power to prevent electrical shock. Originally, the only GFCI outlets we had were in the bathroom so when this happens, I typically check the outlets there first, and hit the Reset button on the outlet after finding and correcting the unprotected cord end outside.
Back to my wireless mesh network, there are three Wi-Fi routers, the starting point in the living room, the end in the family room, and the middle one is in the bathroom--because of the layout of the house, hallways, and placement of electrical outlets, this ended up being the best placement, albeit a strange one.
So, when the Christmas lights went out, I first checked the GFCI outlet in the bathroom--where the middle Wi-Fi access point repeater was located. The AC adapter is just big enough that it's hard to access the reset button when it's plugged in so I unplugged it. My dad was watching football in the family room and that was on a local channel so he wasn't using the Roku and therefore wasn't affected by the loss of Wi-Fi to the back of the house. I set the AC adapter on the edge of the sink so I'd remember to plug it back in after I had confirmed the lights problem was fixed.
In my attempts at narrowing the problem, I remembered we had updated the outside plugs with GFCI outlets and so the bathroom outlet was no longer affected by electrical problems with the Christmas lights. My troubleshooting then dealt only with the outside outlet until I finished--and by then I had forgotten about the AC adapter on the sink in the bathroom. Meanwhile, my wife had seen the adapter laying there, decided it shouldn't be on the edge of the sink, and set it up on top of the cupboard, nicely out of sight.
We went on with our day, finished watching football, and I did some web browsing and programming from my all-wired computers in the office. That evening we tried watching a movie using the Roku and noticed it wasn't connecting. Puzzled, I checked everything I could think of: my wired computers were connecting just fine, there were no changes to internet parameters, wireless was working (at least from the front of the house), everything seemed in place. I reset and unplugged the Roku multiple times--all to no avail. We had to resort to local channels. The next day, I looked at the problem again, did some internet searching, and found a suggestion that even if everything looks fine, try rebooting the wireless router. OK, I'll try that. With my wireless mesh devices, rebooting means unplugging them for 10 seconds and plugging them back in. I went to the living room and unplugged that one, then went in to the bathroom to unplug the second one--and found it missing! Where was it? I found it up on top of the cupboard and it suddenly dawned on me what had happened!
After plugging the Wi-Fi mesh routers back in, the Roku started streaming movies again just fine. Chalk it up to another reminder to check the obvious first!