I had trouble with this a while back, too. I made the following discovery:
If you are a computer “dummy,” you need to know the following, and I have no clue how this applies to Mac computers or Linux as opposed to Windows ones: Most laptops nowadays are made for wireless Internet connections. So they have wireless devices built right into them. Laptops have been made like this for years, so nowadays when we purchase a new laptop, we no longer have to purchase a separate wireless adapter. These are already included and most likely, your laptop has one and it’s turned on. If you want to turn it off, there’s usually a very easy way to do this, usually a keyboard shortcut. You might see a symbol on an F key that looks like either an airplane or a signal tower perhaps with waves around it, or just a dot with widening waves. On a Windows computer, press and hold the F key on the bottom of your keyboard, then while holding this key, press the numbered F key on the top of your keyboard, and this turns the wireless adapter on and off. Sometimes you can just press the F2 key by itself. Sometimes, it’s a different key. If you go to your laptop’s manufacturer’s site you might be able to download a manual which will give you instructions if you have any doubts. You can ask a computer guru or a reference librarian. Or if you live near a computer store that sells the same brand, go in there as if you’re interested in buying, then casually ask the most enthusiastic salesperson how the wireless is toggled on a similar model.
(BTW: “Toggle” is one of those funny words meaning a switch that confuses the heck out of the average person. Why? Because you never know if it’s on or off. A caps lock button is a toggle switch unless it’s like the old typewriter caps lock switches that you pressed and they actually stayed down. Nowadays, the caps lock key doesn’t change at all after you press it unless you’ve got one that does you a favor by lighting up.)
I’m not a computer “dummy,” however, a while back when I connected my Windows 7 laptop to a wired connection, guess what I forgot? Yep, you guessed it. I forgot to disconnect the wireless connection. So both were running at the same time. I wondered why on earth my connection was so darned slow! This went on for a few weeks before I caught my error. I was rather thrilled when I realized my mistake and was easily able to correct it. You can bet I was laughing my butt off at myself.
Some computers won’t allow both at once. Actually, I’m surprised that this one did. Looking back, that’s most likely the reason my Windows 8 machine never accepted a wired connection. It was because I wasn’t disconnecting the wireless connection first.
In conclusion: If you fail to connect your laptop to the Internet via an ethernet cable, be sure that the laptop is not already connected to the Internet via a wireless connection.
This is only one of my many amusing “tech” experiences and it may not be the same as anything you will go through. I just thought I’d share.