The following applies to Mac OS 7, 8 and 9.  One thing that only Mac users were accustomed to back in the 1990's was rebuilding the desktop.  This allowed to solve some benign problems on your Mac that could soon become so annoying that it could ruin your day.  Symptoms that would benefit from a desktop rebuild are:

• Files and applications icon replaced by a blank/white sheet
• Documents that cannot be opened anymore, because the Finder complains it cannot find the application program that created them

This happens because the Finder's association between documents and the corresponding applications that can open them relies on 2 invisible files named "Desktop DF" (desktop file) and "Desktop DB" (database) stored in the root of each volume on your Mac.  After months of using your Mac OS and adding applications, this desktop database can get bloated with changes to a point it actually gets corrupted or incomplete.  Power outages can also cause this as well as an application program deletion or migration to another volume.  When this database contains wrong information, it confuses the Finder as to which application program to launch for a specific kind of document (that document has a file type/creator code that is internally tied to an application program) and it consequently also causes the wrong (or no) icon to be displayed in the Finder.

To resolve this issue:

1) Reboot your Mac and before the startup screen finishes loading all the extensions, hold down OPTION KEY + COMMAND KEY together until it asks you if you want to rebuild your desktop, to which you click YES, obviously.

2) If you have more than 1 volume, continue to hold down OPTION KEY + COMMAND KEY together before the process ends, so that it asks you the same procedure for the next volume, etc... for all your volumes you wish to rebuild the desktop.

3) OPTIONAL: If you'd rather automate this process and go unattended or if you have one of those 2 keys broken on your keyboard, you can instead use a small app such as Trash•Desktop 1.3.1 to trash your "Desktop DF" and "Desktop DB" files.  You can then reboot and it will automatically start rebuilding your desktop.