Is a DS record present at your registrar?
You need to remove any pre-Cloudflare DS records at your registrar to update your authoritative nameservers. This will disable DNSSEC and allow Cloudflare to resolve your domain name.
Do the nameservers at your registrar exactly match the values provided by Cloudflare?
If the nameservers in your registrar do not exactly match those provided by Cloudflare, your domain will not resolve correctly.
Are additional nameservers listed at your registrar?
You should have only Cloudflare nameservers listed at your registrar.