Pause Cloudflare, or set the DNS record to “DNS only” to bypass Cloudflare so you can check the underlying origin server/host issue. Once fixed you can re-enable Cloudflare again.
“DNS only” means requests are not going through Cloudflare.
If the site works ok in that state, but not when proxied then there is some problem for Cloudflare to connect to your origin, or your host requires seeing their DNS records returned instead of Cloudflare proxy ones.
Ensure that you are not blocking any of the Cloudflare proxy IP addresses here…
When using the proxy, use only “Full (strict)” or “Strict” and not “Flexible” or “Full” to ensure your users’ connections are fully secured.