After changing the DNS servers, https disappeared and the browser warned that there was no certificate. What to do?

Question

Domain Name System

I decided to put the domain on a site with a different domain name, I just changed the DNS servers. It took about 3-4 hours, everything was successfully glued, but the certificate that was on the HTTPS domain was gone.
And why in general should it warn about something if a lot of sites are on HTTP and nothing, no warnings.

Answer 1

I decided to put the domain on a site with a different domain name ... the certificate that was on the HTTPS domain disappeared

I decided to move to another apartment, everything is fine, only for some reason the keys no longer fit the locks in the door. What to do?
Of course, you need to get/buy a certificate for the new domain before trying to open it with https://.

Answer 2

I issue the SSL certificate for the domain. You changed the domain and didn't change the certificate. Are you out of your mind?