The problem with having a canonical answer with that kind of problem is that there are way too many variables to take into account. It's Intel, Nvidia, ATI or something else? Do you use open or closed drivers? What version? Version of Xorg? What modules load? Do the modules fail to load? Large etc.
You can't have a canonical answer based solely in 1 symptom, but in several quite common ones. There could be million plus one reasons why the resolution is bad, and each of those reasons deserves their own question/answer.
Note: there are a multitude of "canonical questions" that are super-mega-dumps of lot of information that may or may not work in your case, I'm totally against them and in fact I got one of my questions reopened on SOreopened on SO and here on AU because the specifics of my question didn't get solved with the current answers on the canonical question, the nature of the question itself is radically different and the solution proposed wouldn't work if you didn't had exactly the same problem.
