Assuming you didn’t delete them, they’re still there! Likely someone (ok probably us) was logged in as the admin and using the handy search box just above the domain listing under the General -> Domain section.
While the search box is a great tool for finding a domain quickly, it saves the last search, even after the admin logs out. This can lead to some confusion the next time someone else logs in as the admin user. When they check the domain list they will find only the last search result listed instead of the full list of domains. If this happens to be you, don’t panic! To the right of the search box is a ‘Show All’ link which will clear the search and list all domains again.
This issue pops up at least a couple times a month for both new and experienced Plesk users. We hate to see all the panic and stress it causes—-even if it is short-lived!