I followed instructions and it made my site go down for 8 hours

I received an email that stated the following:

In order to get MYSITE running on Cloudflare, you need to update your nameserver (NS) records to point to Cloudflare. This critical final step only takes a few minutes.
Update your nameservers
Login to your registrar
[Find instructions for your specific registrar]
Update your current nameserver records with the ones we have assigned to MYSITE:

  1. NAME.cloudflare
  2. NAME.cloudflare
    Only these two Cloudflare nameservers should be listed at your registrar. All other nameservers must be removed.

Once I completed the instructions, my site was down for 8 hours. Why?

Domain?