If you chose a free .onuniverse.com domain:
If the app quits or you exit before publishing your site. Your domain is returned to the database, but it takes 1 - 2 hours to refresh and show up as an option to choose. If you are set on a domain, just wait and try again.
If you had purchased a custom domain before the app quit:
If you completed the steps to purchasing a custom domain before the app closed, the domain is saved to you, but it does not a have a site to attach to.
Contact Universe support. You can create a new site using a free .ouniverse.com domain and we will connect your custom domain.