I think (not a technical person) that my SSL certificate has expired on July 10, 2021.
Normally it would auto-renew, but for some reason, it hasn't and now my site seems to be down or at least the backend is.
This site runs through Cloudflare.
I have disabled a number of WordPress plugins to make the frontend working.
Again, I think it's the SSL certificate but, again, I'm not a very technical person.
The website https://culturematters.com shows incomplete pages and sometimes (?) it looks all crooked because style sheets are missing (e.g. images missing) and the backend is not working at all.
When you proxy your site through cloudflare it allocates a certificate to use automatically, but you have also at some point configured your web server to have a certificate using certbot-auto. This would let you access your site using https when not using cloudflare.
Note that even if your server certificate expires your Cloudflare proxying may continue working depending on which SSL settings you have enabled in Cloudflare.
The problems loading stylesheets are because of your content management system settings and are not related to renewing your certificate. It looks like you've maybe fixed that already.
Sorry no I'm not familiar with your system but I would recommend if you're not a Linux administrator yourself that you hire someone on a casual basis to just check things over, document things and do system maintenance like this.
I'm assuming this is for a business. If you don't have someone who can do technical administration it's a risk to your business continuity.