The domain might have been deleted from your account.
This can be the case when the domain has expired. Please check the notifications in your account.
We also send email notifications in regard to domains that will expire soon.
For most TLDs, there is a grace period after the deletion of a domain in which it is possible to restore the domain.
Please see our Wiki: https://wiki.hexonet.net/wiki/Main_Page
Please note that if you just restored a domain, it takes time for it to appear in the domain list.