Hello,
what operating system do you use? Did you get a message which file it was? If using Windows this is the easiest way to do this:
Then go to your encrypted folder, right click the file and finally click on show unencrypted in the boxcryptor section of the context menu. This way works vice versa. You can also go to your drive and select show encrypted to get the corresponding file in your .bc folder.
Best regards
David