As you can see in the title, I'd like to increase my screen's resolution(which dicreased on its own!), but when going to displays, screen can not be recognized and it's unknown. It may be too easy to do, but I'm new here. Thank you.
Go to the 'Additional drivers' app and see if you can install drivers for your graphical card that are a little bit more adapted.
Anyway, please edit your question to indicate what monitor you use, what graphical card you have and which version of Ubuntu you are running, the latest being Ubuntu 11.10. Please also specify what you mean by "decreased on its own": between what moment and what moment did that change appear and what have you done in between? Welcome to Ask Ubuntu, by the way!