This happens to me several times a year. When it happens I have to call or do an online chat with them and explain that my FTP uploads have ceased. Every time it's something on their end. It usually takes an hour or two to start working after they reset whatever went wrong.
On another note, I have noticed that lately it's taking 5 or more minutes for what my FTP program uploads to show up on my site. It used to be almost instantaneous. I am considering moving over to another web host.