Do you have an exclamation mark on your images?
Please follow the below article if you have any exclamation mark -Adobe Photoshop Lightroom Help | Locate missing photos
All the adjustments you make using Lightroom are stored in the catalog. They are not applied to the images. So if you look at those images outside of Lightroom you will not see the adjustments that you have made. It's necessary to export copies of your original images in order to have an image that includes all of your Lightroom adjustments.
Photos can become lost if you move them using your operating system rather than Lightroom. When images are imported into Lightroom, their location is stored in the catalog. If you subsequently move images using your system browser then Lightroom won't know where you have moved them to. Is this possibly what has happened in your situation?