If you don't have access rights in Boxcryptor anymore, it is not possible for you to "restore" them. Possible solutions for this situation can be:
1) You ask any of the other users with access, to grant you access again.
2) Because the Boxcryptor permissions are stored directly within the encrypted file, you can regain access by restoring a version of the encrypted file before your access has been removed. This will also restore the file contents.