What happens with my seo when i change domain?

1917

Hello, my client asked me to buy him a new domain due to the fact that there was an error in the old one.

I added a new domain, changed the site texts for it, and it instantly disappeared from the search engine results.

On the old domain, the site was on the first page of search results, there is no search results for the new domain.

Month has passed, but there is no indexing by keywords.

What factors affect to seo when changing host and building new site?

My client panicking now, but I don't know what to do.