What should I do if the created app is not showing on the lightning app builder?
I have made the app for a few days which is not visible now. May I know the possible reason? The app is accessible from the app launcher but not from her.
Your screenshot is of the Lightning App Builder setup page which tells you what it's for at the top The Lightning App Builder provides an easy to use graphical interface for creating custom Lightning pages for Salesforce Lightning Experience and mobile app. Lightning pages are built using Lightning components—compact, configurable, and reusable elements that you can drag and drop into regions of the page in the Lightning App Builder.
Hence, you wouldn't see your actual app there. You want to go to Setup --> Apps --> App Manager There you will get the Lightning Experience App Manager page where you will see your app (or create a new one). You may also be interested in the Create and Customise Lightning Apps trailhead which goes over this.