Hard to say. Magic Dosbox itself doesn’t erase anything, not even when you uninstall the app.
My best guess is that you’re using a cleaning tool, like e.g. Clean Master, and accidentally erased the MagicBox directory. These tools are extremely dangerous, because they tend to destroy directories they deem unnecessary. It has happened once in the past, I believe, when Cleam Master was the culprit. Therefore, MagicDosbox has a Backup function since the last update, to prevent complete data loss. That’s probably too late now for you, but a hint for the future. The backup has to be triggered manually, because the backup file can grow very large. An automated solution would probably piss of people with low internal storage. 😉
Another possibility would be data corruption on your sd card. But that’s highly unlikely, if everything else is still working.
On the off-chance you simply messed up your app settings: perhaps you have simply moved the default data location (check Global Settings in mDosbox). And check your internal sd drive, there should be a folder labelled “MagicBox”. inside, where your profile data is stored (in “Games/data”).