That depends on how you had the notes. As files? Or as entries in an app like Notes?
First, you'd have to root thew phone.
Then, if the notes were in files, you'd need an app like DiskDigger undelete to undeleete the files.
If the notes were in a database kept by some Note app, you'd need a SQLite editor. You'd have to find the right database file, figure out which field indicates "active/deleted" and change it to active for he notes that are marked deleted.