What are the causes of "No Internet Connection"?

[Josh Bommer]

About an hour ago, I went through "heck" just to get my internet connection working again. I managed to do so, but can anyone give me a list of the stuff that causes "No Internet Connection"?
Also, does the things below cause "No Internet Connection"?

1. Downloading video games, music, videos, and software online.
2. Staying connected to the internet every single day and night.
3. Disconnecting from the internet manually.
4. Leaving the PC on (not shutting it down when you're done with it).
5. Disabling and enabling your network connection back and forth over and over again (trying to fix it).
Thank you so much for your answers!

[Bomber]

oh god lol... ummm ISP servers being down... Modem having problems, router having problems (if you use one), IP conflicts... uhh.... DNS server not working.. uhh... faulty Ethernet cord... Wireless card not working properly... Ethernet port not working... specific router ethernet slot not working... want me to continue? >.> theres LOTS of things that can cause "no internet connection" the most common problems though are
IP conflicts
Modem not working properly
Router not working properly
uhh DNS servers not working...
these are the most common problems for me anyways... usually a simple restart of the router or modem will fix these problems.

viruses can sometime cause internet connection problems.... this is a really common one....


*EDIT*
1. Downloading video games, music, videos, and software online. A: I actually had this problem. I had very crappy internet, I downloaded way too much, so my modem would just disconnect and refresh itself... very annoying :/
2. Staying connected to the internet every single day and night. A: I think at some point your modem does need to completely refresh, my modem goes through this every once in a great while, connection loss doesn't last long.
3. Disconnecting from the internet manually. A: No.
4. Leaving the PC on (not shutting it down when you're done with it). A: I've had my PC on all day and night for the past... month now... I've seen a few internet connection problems, but I don't think it was because of me leaving my PC on 24/7.
5. Disabling and enabling your network connection back and forth over and over again (trying to fix it). A: No, lol as this is a common way to fix it, usually it works.