Remember your FTP username is all lowercase and a maximum of 8 characters long. Example: if your Control Panel username is JohnMiller, then your FTP username is johnmill
If you just turned
on the "FTP SERVER" in your Control Panel, you should turn it
back off. To
do so, log in to your Control Panel, click on FTP/USER ACCOUNT on the
left side. Then
click on FTP underneath that. On
the right side, if you have turned on the FTP Server, it will say FTP Vhost at the top. On
the second row down it will say Name
of Server and will list the name. Click
on the trash can 
next to that to disable the Virtual FTP server. You
should now be able to log in with standard FTP.
Check your FTP setup. If you can log in but can't stay connected, turn on Passive Transfer Mode in your FTP software. For additional setup, see this page.