To retrieve deleted files on a Chromebook, open the Files application from the Launcher and select Trash from the left-hand sidebar. Look at the list of files and click on the file you need, then select "Restore from trash". You may also look in the Recent tab of the Files application for any files you recently opened, but note that it will be different from the deleted files folder. If you deleted a file from Google Drive, open it in your browser and check for the file in the Trash tab. The trash tab on Chromebook usually keeps deleted files for 30 days before deleting them permanently. If the lost file was located on an external hard drive or an SD card and is now missing, you may want to scan the disk using recovery software on Windows or Mac OS. Notchox is one such program that may help.