Problem on a elgg site: need help

atm I can't access in admin pages (white page) , in settings pages (white page), and all over the site there are no images (only default elgg thumb ) for profiles and no images for files. then when I click on Logout, an error message says "Action is not define in the system".

I updraded also too elgg 1.7.6 and using now default mod/plugins.

Upgrade.php 1.7.6 runs fine but all this problems are still there.

maybe something it was going wrong enabling/disabling plugin. Do you have an idea to resolve it?