Hello, and thank You for your answers.
1) Since the site is in a developing phase, it was an habit for me now and then to visit the permalink page just to save the setting without changing it (I was told this could help wp working without cache issues).
2) I didn't changed the URL of the site.
3) Yes, I installed a plugin yesterday but in this phase it happens that I install and/or uninstall plugins, but frankly I didn't notice problems with this.
1) This is not a production site since I'm still developing/testing it locally wuth wamp/xpsp3. However I have wp installed within a subdirectory like "[my-site-domain]/name-of-subdir/".
I tried to change the theme folder name, it didn't worked the 404 remains.
Any hint would be appreciated. Could you tell/know how I could try just to enter dashboard?
Thank You very much