Is the file still present in the encrypted folder?
If you're using BoxCryptor with Dropbox, you could try to restore a previous version of the .encfs6.xml configuration file located in the root of your encrypted folder (e.g. Dropbox\BoxCryptor\.encfs6.xml) and enter your old password after restarting BoxCryptor. This file contains all information required for decryption and gets changed when you change the password.
