Most likely this is because the original image is missing. Perhaps it's on an external drive that isn't connected. A missing image would normally show an exclamation mark, but if you created smart previews it does not. That makes it easy to miss. Look underneath the Histogram. Does it only say 'Smart preview'? It should say 'Original Photo' or 'Original + Smart preview'. You can edit smart previews in Lightroom, but you cannot send a smart preview to Photoshop. You do need the original for that.