How is your network connected to the internet? If, in fact, nothing -- about your client, server, or intervening network equipment that you control -- has changed, then it must be either the ISP that you are connecting from or the ISP that your server is connected to.
It's possible that your ISP has suddenl started blocking FTP ports. And this is the most likely possibility if "all of [your] users have experienced this".
However, if the assumption is incorrect that nothing has changed, then you might be having problems with passive vs active connections, and possibly "extended passive" issues as well.
I would suggest using the terminal FTP client ('ftp') and start it with
ftp -d user@your-server-address
which enables debugging. If you have trouble connecting, you might get some info. Next try with the "-p" flag (for passive), the "-A" flag (force active), and finally, if you get connected but can't transfer anything, try giving the command epsv4 at the ftp> prompt (toggle the use of the extended EPSV and EPRT).
I presume that your Web FTP server is running on some port other than the standard FTP port, which is why you can get through.
If possible, I would suggest changing the external FTP port on your firewall and map it to the normal ports on the inside pointing to your server. The tricky thing about FTP is it actually runs on 2 ports: 21 for control, and 20 for data.
The other thing is that FTP is inherently insecure. You should use sftp instead.
