Did you happen to change your url address from http to https? If so, on some browsers, you will have to type in the https in order to have your icons instantly restored. I'm not sure why, but this worked for me.

I'm certain that an experienced web developer could tell you why but... that also excludes me...

However, I can also tell you that if your computer has too many programs or web pages open, your resources (ram) may have become used up and some graphics will not display. The Safari browser is famous for this.

Good luck!