Let’s Encrypt certificates always expire after 90 days. You can check the validity of the certificate that you’re accessing in your browser. For example, in Firefox l can click on the padlock, then the > arrow, then More Information, View Certificate, and I see that the current certificate for this forum site expires on “October 28, 2018”.
With Chromium, I can click on the padlock, then “Certificate (Valid)”, and I see that the certificate expires on “Sunday, October 28, 2018 at 5:00:05 PM”. (This is nice—this had been moved into the web developer tools but I guess now it’s back in a more convenient location.)
The reason I said “may” above was that I didn’t understand from your message from your hosting provider’s support whether the provider had fixed the certificate this one time or had set it up to auto-renew on the provider’s end (which you might be able to ask explicitly about since it would be useful to know).
Having a certificate expire without replacing it will cause all HTTPS access to the site to break until the certificate is replaced. But the Apache configuration shouldn’t be specific to an individual certificate.
I think we’ve been unreasonably slow in saying this, but basically the best answer is
providers should perform the certificate request and renewal for you as part of your service plan, or
you should obtain the certificates yourself with a tool like Certbot if you’re the system administrator, and configure them to auto-renew, or
you should use a web server application that has certificate requests and renewals built in (currently probably Caddy)
Having users obtain their own certificates in shared hosting environments or when they’re not familiar with system administration is, as you’re pointing out, a recipe for disaster or at least annoyance.
I’m not sure how we want to assign responsibility in this case. One answer is that we need clearer documentation about using HTTPS with WordPress (which might come from the WordPress developers or community); another answer is that we need clearer delineation of what different kinds of hosting providers are and are not willing to accept responsibility for; and another answer might be that you’re using the wrong kind of hosting provider for your needs because you might want more proactive support, or more hosting plan features, than your current provider(s) are prepared to offer.