Depends what have you already tried to fix it?
Can you post your domain name here so we could check?
Have you already changed your nameservers to Cloudflare ones at your domain registar?
Have you tried following the steps from as written in the article from below?:
Have you allowed Cloudflare IP addresses to connect to your new host/origin?
Maybe you need to allow Cloudflare IP addresses to connect to your host/origin server:
https://support.cloudflare.com/hc/en-us/articles/201897700-Allowing-Cloudflare-IP-addresses
Have you got an SSL certificate for the needed hostnames of the domains?
Can you check what have you got selected under SSL tab at Cloudflare dashboard? Is it Full SSL or something other?