I've switched themes, and none of my posts have featured images anymore.

Sometimes, customers will begin using our themes and have previously not been assigning Featured Images in their posts. All of our themes rely on Featured Images for features in the blog feed and in some of our custom widgets. Plus, it’s a good idea to assign a Featured Image anyway, as this is the image that will be used when your content is shared across social media, or via RSS.

If you have a large bulk of posts without Featured Images, you don’t need to assign a photo to each post one by one. Instead, use this plugin to run through your posts and automatically set the first image as your featured image. You can deactivate and delete it once you’ve run the process!

After running, you may wish to run the Regenerate Thumbnails process to resize any images that do not properly fit their container size.