Lost photos in Photoshop Elements 13 after moving the photos to an external drive.

I am an Adobe rookie.  My computer was out of storage, and I moved a large number of photos to an external storage device without thinking about the Photoshop Elements 13 edits.  Now, in Photoshop I get the dreaded "?" indicating lost files.  How do I reconnect PS 13 to the photos on the external drive?  Thanks!