In whm you can suspend domains by using (Manage Account Suspension), after suspending a domain you can find that domain inside (List Suspended Accounts) there you can un-suspend any domain easily. But some time due to any reason that domain not comes there or vanished from there. So what you need to do?
That time website will show you a suspension page but actually there is no suspended site in List Suspended Accounts, there you can try suspend and un-suspend again but this will say account is already suspended. So why that domain is not listed under List Suspended Accounts page! Some times server have any issue / load is high / any service is restarting / anything etc may cause suspension process incomplete. So website suspended but its status not saved properly inside List Suspended Accounts.
Login to WHM as root user
go to /var/cpanel/suspended
There you will see all accounts which are actually suspended, you can simply remove them from there or you can rename the files inside this folder. And that website will be activated properly. Now you can suspend it again if needed 🙂
Hope you got the info about how to solve this issue.