Sites down

Hello community,
This is urgent!
I have a domain with my one DNS used on client´s domain (I do hosting reselling). So my clients domains have ns1.mydomain.com, ns1.mydomain.com.
Y activated cloudlflare to my website and yesterday we changed the DNS on our website to the ones of cloudflare.
Today some of my clients (with the ns1.mydomain, ns1.mydomain) cannot enter their site.
They get
DNS_PROBE_FINISHED_NXDOMAIN
and
err_name_resolution_failed
What do I have to do?
Change again to the old DNS, before cloudflare?
It is kind of urgent as there are several sites down.
Please help!

Without knowing specific domains, it’s difficult to troubleshoot, but if “mydomain.com” is your main domain that you’ve switched to Cloudflare, those “A” records for ns1 and ns2 need to be set to :grey: