This is a curious problem. I have a site in which I'd installed k2 (Joomla 3.1+ and Yoo template), but we decided after the fact that it would be better to use joomla's core content system. Yesterday I did a full backup, then disabled all k2 extensions. This broke the public site - all, absolutely all content under the main menu, went away on all pages, not just article/content pages.

I backtracked and found that it is the "K2 System" plugin that is causing the problem. Disabling everything else k2 related including k2 administrator caused no problems. Ideas please

Did you also changed all menu items to point to com_content pages and not K2's?

Yes, and like I said, this affects all pages including those pointing to non article pages like Contact Enhanced (com_contactenhanced). I have removed all trashed menu items as well, no remnants.

Any news in that, i have the same problem...besides, unpublishing K2 system plugin breaks also the look of the site, the sidebars dissapears

No, in my case and with the site that I was working on, this last feather was the one that broke the camel's back. The horrible combination of K2 JavaScript and CSS overload combined with core Joomla JavaScript and CSS was teetering before this last problem occurred.

I not only dropped K2 but also dropped Joomla and created a custom CMS to manage the highly structured data administration which I'd hoped that K2 extra fields could handle. Public pages also now load fast without employing various Joomla performance steroids.