UPDATE:
I remembered that I could ps aux and then sudo kill 7875 to stop the process.
And yay, the certificate renewed successfully!
However, on https://www.ssllabs.com/ssltest/analyze.html?d=codebuddies.org, the new certificate still isn’t recognized, and the site is still down.
Is it normal for there to be a couple hours’ delay when a certificate is renewed?