What sort of server is it hosted on? Is everything else functioning normally?
Assuming that it's some sort of managed server, have you contacted customer support?
I tend not to use ftp myself - don't ask me why, I just don't really like it - so I'm not an expert, but as it stands it's a bit of a vague problem.
Personally I'd probably try doing a port scan of the server with nmap to check that the target ftp port is actually open, although some people regard port scanning as an attack...