where you start to really depends on the nature of your problem.
In every case it could not harm to power off the computer and remove the mains cable or power supply.
If you have a laptop and you can do it easily, remove the battery.
Leave the computer switched off for several minutes.
If your problem is any way related to the Internet, I would suggest the you power off your router as well.
When you are ready, replace your battery (if you took it out) reconnect your power supply , or power cable.
Restart your router and wait several minutes until all the lights on the router are on and your connection is working.
On restarting the computer try to connect Internet using web browser. Preferably using different browser from the one that you normally use as your issue could be a browser problem.
If You can not access the web with your alternative browser to check your network connectivity. (acccess the internet)
If you are using wireless check the bar near to the clock (situated the right of your display )
to ensure your wireless is connected.
If it isn’t check that you’re not running aeroplane mode and that your Wi-Fi card has not been turned off.
You may also like to connect ethernet cable to check connectivity using an ethernet cable.
It’s a bit like Lego working in reverse order. You need to be sure that you have an Internet connection from your router first of all and then do the other steps I’ve indicated above.
I suggested using another browser from the one that you normally use when testing because browsers save all kinds of information regarding your web activity (this is normal).
Furthermore, browsers may have what are known as extensions which provide some functionality that is not included in the basic browser. One such extension which I use on many people machines is called ad block plus and is used to reduce the number on adverts you see.
There are some more advanced troubleshooting techniques that can be used if the above fail, these include resetting all networking within Windows but I do feel that they are beyond the scope of this article.
I hope it helps.