If the billing is paid in full and not late, there may be a security ticket that must be resolved first.
Either way, please contact us via phone or live chat at your earliest convenience.
If your reseller customer is suspended, you can unsuspend them in WHM. Click Suspend/Unsuspend Accounts in the left menu, highlight the domain, and click unsuspend. If you cannot, then there is a security ticket that must be resolved first.