That is your browser displaying the alternate text because for some reason the image could not be displayed. This is a normal behavior for whenever an image can't be displayed. Without alternate text set in the html, Firefox will normally just not show any image or text. In IE, this is when you would see the red X.
Either something happened when your browser tried to request the image be downloaded from the server, or something corrupted/removed the locally cached version that your browser thought it had.
Does hitting refresh make it work?
