check if you still have a file NppFTP.xml on your system.
This is the file where username and password are stored.
Depending of your npp configuration it must be copied
to
%APPDATA%\Notepad++\plugins\Config\NppFTP\NppFTP.xml
or
INSTALL_DIR\Notepad++\plugins\Config\NppFTP\NppFTP.xml
If you still have access to the webserver from another machine,
than you can snoop the network traffic with tools like wireshark,
if you have used ftp protocol only, of course.
Cheers
Claudia