It might happens you start getting cpanel/whm default page for all the domains.
Follow the steps in order to resolve the issue:
Run following command:
Option 1:
/usr/local/cpanel/bin/userdata_update
This command helps to restores the missing alias in the userdata and merge it with your existing data.
/scripts/rebuildhttpdconf
This command helps to builds the httpd.conf against the merged userdata.
Option 2:
If still you get cpanel/whm default page then check name server ip’s.
Update name server ip’s from whm –>> name server ip.
Thats it. The domains should work fine now.