Hi Everyone, I have had this problem intermittently. The first time my host, inmotionhosting, helped me resolve the issue. However, because i did not know what they did i could not fix it again.
Today I had the same problem and managed to get a guy called Tony. It turns out that the .htaccess file under public_html file had a line in which prevented the icons from showing. When he deleted it the visual editor icons immediately showed.
Although this was fine on my other websites. It would appear that the .htaccess files sometimes could get amended or a new one created. Check all your .htaccess files.
BTW, i am no techie. However, i specifically registered because I could see many people struggling with this.
Hopes it helps for many of you.
Iggy