Your certificate must have expired at this point. Considering that your site is still reachable you must either have the completely insecure Flexible setting on Cloudflare or the less secure Full setting. If it is the former case you definitely need to switch away from that setting.
If it is the latter case it will explain why the page is still reachable but you should still renew your certificate and switch that setting to Full strict. Only that will guarantee a properly secure site.
As for the certificate renewal, who had that server certificate issued first? Your host? If so, you should contact them, respectively they should have renewed it automatically.