The alias "..." can't be opened because the original item can't be found.

KubeShark wrote:

This shows up. I've tried restarting, and don't have a backup.

In your first post, you said: "I recently copied a large number of files from my desktop to a folder..." Where did you place the folder?

If it is in one of the locations listed, i.e. Documents etc, you should be able to expand the location and find the files.

KubeShark wrote:

The folder is on my desktop, but when I select desktop and go to the folder, if I click on the file I'm looking for, it turns that file into a folder than contains the same things as the original folder has. If I click on that again, it crashes finder.

Open the folder on your Desktop. Can you drag one of those files back to the Desktop?

KubeShark wrote:

Yes, but when I try to open the file after dragging it to the desktop, the same error appears.

Are these files displaying the "Alias" arrow icon as depicted in this image?

If so, Select the alias, then choose File > Show Original, or press Command-R. (macOS Sierra: Create and use aliases)

Did you move these files to/from any external drive?

Here are a couple other posts which discuss alias problems:

1. The alias can't be opened because the original item can't be found

2. Alias can't be opened?

Another possibility is a corrupt Finder .plist file. The post byEric Root in Finder won't relaunch even after force quit outlines how to refresh the Finder.plist file, but requires a backup.

KubeShark,

Can you select "Fix Alias..." listed in that dialog?

KubeShark wrote:

Yes.

What happens when you click on "Fix Alias...?"

Have you re-started your computer?

Do you have a backup?