FTP Connection Problem

I have a connection problem with the site.
I am using a standard way with FlashFXP (Port 21). It was fine while connecting to the site until one day I don't know why it doesn't connect anymore.
I use the same user/pass as in cPanel and it doesn't work, I also tried to connect with other programs and it says the same thing "Connection Failed (Connection lost)".
If anyone knows why, or if cPanel can help how to fix it just reply.
Thanks in advance.