Thank you all for your replies and for the nice welcome.
I turned off my firewall and problem solved, but I have a new one:
Before knowing the solution, I went to the CPanel and created a subdomain named ftp and an FTP account called the same way in order to get FTP access to the subdomain. But then I erased that subdomain, and now there is no answer on ftp.cotisaweb.com.
Should I make that subdomain again?