If the main domain of published website is changed then the domain should also be updated in builder to a new one. Otherwise builder will see the new domain as a new website and will offer to choose new template instead of loading an existing website.
To change the domain in builder take the steps below depending on your brand type:
For Cloud brand:
Contact us in support system by creating new ticket and provide us old and new domain names. Our team will update domain in site builder and the website will become editable again.
For On-Premises brand:
Use builder API to rename website in your site builder installation.