Sounds like the apps' database got corrupted and then Android deleted it (the database), including all message text and folder settings which the app stored there.
It does happen, very rare, but very unpleasant for sure (and the code in Android to detect and delete corrupted databases is from 2010, long before my app even existed).
To restore the "mapping" of "special" (Sent, etc) folders to server folders, please do this:
- Long press the account
- Options and folders
- Folders
Let it refresh
- Find the Sent folder, tap its name, choose "Sync with [ Sent ]".
- Repeat for Deleted and Drafts, and possibly Spam and Archive.