I just got a call about 20 minutes ago from a client saying he couldn't log into ftp... when I tried, it won't work for ALL accounts on one of our ip's....

We're using cpanel and WHM... we've retarted ftp, synced passwords, etc...

Did you check with your host? It might due to router problem which we do encounter before.

Besides, check that you are not using PUREFTP. If you do, change them to PROFTP instead.