Hi Luigi,
if you were using https://github.com/gpg4usb/gpg4usb, note that this software is not maintained anymore (last stable release 2016).
Your secrect key needs to be saved somewhere to be able to re-import it.
If you have not saved your secret key, there is no way to recover it (of course).
Where did you save or backup your secret key?
(I don’t know where gpg4usb does this.)
Best Regards,
Bernhard