I can't see my Site

If you are unable see your site in your My Sites list - https://sites.google.com/new then either:

You are not signed in with the correct account - ie. an account that has editor or owner access.
or
Your Site has been deleted

Make sure you're signed in with the same account that you created the site with and check your web browser's browsing history to see if you can get back to your site from there.

If your site is a Google Workspace site (address starts sites.google.com/a/DOMAINNAME/..) you need to sign in with your @DOMAINNAME account. If you cannot remember what your account details are ask your domain administrator. If you are the domain administrator and cannot remember your domain admin sign in details you can reset them at:

https://www.google.com/a/DOMAINNAME/ForgotAdminAccountInfo

Deleted Sites

Google Sites (New Sites) are stored on Google Drive. Sometimes when cleaning up their Google Drive, users inadvertently delete their Site(s). Note that when a Site is deleted the owner account will receive an email of the form:

Check your Google Drive Trash/Bin to see if the site is inadvertently deleted.
This search will find any deleted sites https://drive.google.com/drive/search?q=type:site%20is:trashed .

See this help topic, Delete and restore files in Google Drive and recovering files.

If the Site file has been permanently deleted (which happens automatically 30 days after it is deleted) files is usually gone forever. But there may still be a small window of time to recover files by contacting a Drive Specialist. If using a Free Google Account or Google One Account see Contact a Google Drive Specialist, if you are a Google One Member you can contact Google One Support, if you use a G Suite Account it is recommended you Contact the Account Administrator. Also, this post File Recovery Best Practices may be helpful.

Classic Site deleted

If your Site was a Classic Site it may have been deleted as part of the migration to New Sites. See Migration Issues II for more information