Double check your user name and password using CPanel -> FTP accounts. You need to type the user name like user@domain. Make sure your FTP client isn't blocked by software on your computer (firewall). If these things do not work, it may be a temporary network outage. If you can view your site on the internet, your FTP should work.
A quick way to check if your information is good without the FTP client (if you're using Windows) is to do this:
start menu -> run: cmd.exe
type "ftp" (enter) at the prompt: C:\> ftp
type "open ftp.domain" (enter) (replace domain with your domain): ftp> open ftp.domain
type your user name (enter): user: user@domain
type your password (enter): password:
And it should be able to connect. You can exit by typing "bye" (enter) then "exit"(enter)