Can't connect to ANY minecraft server?

I've been playing multiplayer minecraft since the 1.4.2 update. A couple months ago, i think it was minecraft 1.7.6, whenever i wanted to go on a server, it said ''Can't connect to server''. I've tried a hundred of different servers, I'm SURE they work, alot of my friends were on a server, but i couldn't go on it.
Minecraft has recently updated to 1.8. But it's still the same problem. I've searched alot, and there was this thing with firewall, i did it, and it still did not work.
I think it is something to do with my internet connection, because my sister uses the same internet and she has the exact same problem.

Are you sure you are using the right minecraft version that the servers you are trying to connect to are using? I own a server, and we get that issue a lot just because people don't realize they need to be on version 1.7.2, 1.7.4, or 1.7.5 in order to connect to our server (which is 1.7.2). You can easily change it by editing your profile in the minecraft launcher (before you hit the play button). Maybe your issue is as easy as changing your profile (hopefully).

Nope, i've got the same version as all the servers i've been trying to go on.