I apologize in advance if there are any inaccuracies in the way I'll expose my problem... I'm not an IT expert but I'll try to do my best.
So, I'm using Ubuntu 14.04 and the thing is that Firefox (which wasn't giving any problem until now) is no longer loading most of the pages I use to browse the internet, but still works with others (I'd say the major ones like Google for example). It also seems that other applications don't connect to the internet any more, and connecting to a different wireless is useless. However when I boot windows on this computer internet works fine.
This happened right after I tried to set up some PPPTP connections through the network manager, following this step-by-step tutorial.
Other times that I've set up the proxy connections using this very method I haven't come across with any bothers, but this time I wasn't able to connect to the new-added networks.
Before realizing the scope of the issue (that is I wasn't able to load the pages even when I was disconnected from the proxy), I also went on the "proxy network" settings in the "network" utility and set all the new proxy connections as "on". However all other parametres are unchanged, and I don't think that this is really what triggered the problem.
I reckon it must be something related to the DNS, but I know quite a little about that and I have no clue about how to cope the problem. I hope you can help me to fix this.