You haven't mentioned whether your sites are on your own dedicated server, or whether you are using a reseller package?
If you're on a reseller package, you would need to check if your host has suspended their account or not?
Also, when you say your client cannot access the site(s), is there any error page or something coming up?
It could be a number of issues from routing issues from their location (are you able to access the sites?), to perhaps their DNS entries being changed either server side, or their settings with the domain registrar ...
Have you checked the DNS settings of the sites which are "down"?
If they're on an external DNS, then check that their "A" record is pointing to your server ... if you are hosting the DNS on the servers, check the DNS records on your server for the domain names which are not resolving.