Access it how, Phil? Browser (I guess)?
In that case all you need is a new SSL certificate. It could be the code that serves the html pages installed its own self-signed certificate and it's now expired.
It can be regenerated, but you need to know a few details. There's also a possibility that upgrading the code, or calling a reconfiguration in it will regenerate the certificate.
eg here's a page about "how to generate self-signed certificates".https://www.sslshopper.com/article-how-to-create-a-self-signed-certificate.html
It's nothing but a couple of files that resides somewhere in the file system, that the web server reads and uses for encryption.
You can call a command on linux to generate such a certificate.
Alternatively, you can get free signed certificates, eg https://letsencrypt.org/
It may look a bit easier, but you still need to find the originals to replace them, and of course give the certificate authority the same details to generate the cert - eg hostname.
In fact a lot of these require you to generate a key file called "CSR" (certificate sign request) via apache, upload it, and they'll give you the certificate files.
How to find the certificate?
In apache, you just look into the ssl.conf file, which usually resides somewhere like /etc/httpd/conf.d/ssl.conf, it's a few pages long, but will have a line like this:
You need to replace a few files, mind, not just that one. That's simply the public certificate file.
There's also the private key
And the certificate authority file:
They may not have the same extensions, and will probably reside elsewhere, not in /var/www/ nevermind that.
After replacing them, restart apache.
Do take backups of both config & certificate files, in case something gets fucked up!