Which Error message/error code do you get? It’s hard to say much without one.
If you are getting a Cloudflare branded block page, check your Firewall events, Security → Events or Magic Link: https://dash.cloudflare.com/?to=/:account/:zone/security/events. You don’t seem to have Pro, so if anything is blocking it, it might be the free WAF Rules or a firewall rule you configured.
If it’s a non-Cloudflare branded error page, chances are it’s a problem with your origin web server. You can search the community for tips on certain errors too, like 503, 520, 521, 522, 524.