The account gets suspended due to non-payment of billing, the blacklist of IP address due to incoming spam emails to that particular account, etc.. An account can be unsuspended in many ways. Back-end scripts are available to unsuspend an account in WHM/cPanel server. An account can be simply un-suspended via command-line using other cPanel scripts. The command for unsuspend an account through SSH is given below
/scripts/unsuspendacct username