In some cases, file tasks are lost due to file corruption or other system errors. However, file tasks are not the same as the backed up files themselves. They are simply the reference data that contain information such as which files to backup and when. The best thing to do when a file task doesn't show up in Online Backup is to recreate it. To do so, give the task the exact name as before and also select the same source folders. If you don't remember the task name, you can manually find it by looking in your backup folders (DriveHQData folder > DriveHQ WWWBackup > Data).