Okay, good.
Start up ResEdit. If it sits showing a splash screen with a jack-in-the box on it, click that screen to make it go away; it should then show a standard GetFile screen. Navigate to your known-good file, select and open it.
Ignore the window that opens showing a bunch of resource files. Instead, choose Get Info for <name of file> from the File menu.
In the new screen, under the file's name in the top box will be two boxes, one named "Type:" and the other named "Creator:" Copy those two codes down (on paper is fine). Make sure it is an exact copy: each code is exactly four characters long, capitalization is important; codes can contain letters, numbers, and even special characters including a space. Make sure you copy them exactly as they are shown.
Then close out of those windows, choose Open from the File menu, navigate to and open the problem file. In the appropriate boxes enter the Type and Creator codes which you copied down - again, make sure they are exact.
Then Save the changes, quit ResEdit, and see if the file will now open. If it now displays the correct icon, it should open normally. You might need to restart, and/or drop the file's icon onto the icon of the program that should open it.