Here's the stupid thing I did: I deleted many folders inside the icon folder, with root privileges, and now some icons are gone, but not the main ones, the ones from compiz for instance, or some from top notification bar, like the skype one. I looked for days to find a solution to restore the icons folder, or to find the default Ubuntu icons to put them back to the folder, but I have found none.
I use Ubuntu Tweak Tool to switch between themes, icons, or cursors, and it works perfectly, but no theme or icon pack seems to bring back some of the icons like I mentioned before.
I hope there is a way to reinstall ubuntu default icon packs. Thanks in advance ;)