Icons are cached and the cache gets corrupted. This can affect all applications and even files associated with them such as acrobat reader files suddenly not having the acrobat icon.
There's a way to clear the cache and rebuild it.
Exit all running programs.
You need to open up a command prompt with admin privileges.
Press Win +R to bring up the Run box.
Type in cmd and check the admin box.
You'll get a confirmation with are you sure you want to do this.
Click on Yes.
In the box, type the following commands one at a time. Remember to press Enter after each line.
taskkill /IM explorer.exe /F
DEL /A /Q "%localappdata%\IconCache.db"
DEL /A /F /Q "%localappdata%\Microsoft\Windows\Explorer\iconcache*"
When done, close the window and reboot.
If you want to be totally geeky, you can automate the reboot by typing in.
shutdown /r /f /t 00
And even more geeky by taking all the above lines and putting them in a .cmd batch that you can run by double-clicking on when things get stuffed. The issue is rare enough, however, that this may not be necessary.