If you've accidentally deleted some data (and emptied the trash) and then realized that you still need it, or if some kind of bug or sync conflict causes you to lose data, then the easiest way to get it back is to restore a backup. But if you don't have one or don't want to restore data (since it might mean that some of your tasks are suddenly no longer marked as done), then the best way to recover your data is to load Marvin on a device that you haven't used in a while. BUT normally cloud sync would quickly bring this device to the latest state and you'd lose your data again.
Instead, load https://app.amazingmarvin.com?nosync, and Marvin won't sync new data from the cloud, or sync local changes to the cloud. So load that link, do a quick backup of exactly what's missing, and restore that after you re-enable cloud sync (and after Marvin finishes syncing).