Letsencrypt SSL cert expired, automatic renewal failed

I got msg from members that they can't access the site and after some digging, it turns out that the cert has expired sometime today and the acme script failed to renew like usual.
My immediate question is how to install another cert so I can get the site up and running. After buying a new cert, I would have to config the cnf file to use this new cert and I want to make sure I get it right.

Secondly, I'd try to figure out why the cert does not renew.

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I bought a Comodo SSL cert for $8 last night and installed it and get the site back within 30 minutes.
It's a lot more efficient than going through the logs to figure out the issue and try to fix it without any guidance from @eva2000

Free is nice and I still plan to use it for other domains that I do not actively use.