Symptoms

Most of the pages/spaces are not accessible.

Cause

The section "Content DIV" part in the main Layout template was deleted, causing the pages/spaces to be inaccessible.

Resolution

You can try to restore the default layout by referring to the following link:
http://<confluence base url>/admin/resetdecorator.action?decoratorName=decorators/main.vmd

See Customizing Site and Space Layouts for further details.