I clicked "ok" to the prompt "An earlier version of this file exists, do you want to replace?"
This sounds like something you did in the Finder. What exactly were you trying to do? With what file(s)?
The default location for FCE project files & related media files is /users/yourusername/Documents/Final Cut Express Documents. All your FCE projects and clips will be located there unless you manually changed your FCE Scratch Disk location in FCE Preferences.
In that folder there is a folder called Autosave Vault where you will find periodic backup copies of your FCE project files. Pick the most recent backup copy of your project, copy the file & rename it, then open the renamed copy. This should get you back to where you were before.
Also in that folder there is a folder called Capture Scratch that by default contains your source media clips. Look in there to find the files you need to reconnect.
I was trying to make space by getting rid of earlier files that (I thought) had been made redundant by later versions. When I realized that I had trashed the one that had all the media files -- which were apparently referenced but not copied and incorporated by the newer version I pulled what I had dumped out of the trash and put it back on the external hard drive. That's when I got the prompt asking if I want to replace the earlier version.
I followed the steps you outlined, but it still shows all media files as offline.
Is there some way to simply search the external hard drive for the folder (which in the now MIA version is named "Elements") that I need that contains all these medai files?