This usually happens after you've switched to a different browser or computer, recently cleared the browser's history and cache, or restored to the browser's default settings. This may also happen after you login to a different account.
Please note that the code you typed into the code editor is stored temporarily in your local storage, however, all your submitted solutions are stored permanently.
By clicking on the "code" icon 
on the editor toolbar above the code editor, you can retrieve your last submitted code.
Still having trouble finding your code? Check the session management page, make sure you have selected the correct session so you can retrieve the submissions that are associated with that session.