HTML only after upgrade

Nextcloud version (eg, 10.0.2): 12.0.4
Operating system and version (eg, Ubuntu 16.04): Ubuntu 17.10
Apache or nginx version (eg, Apache 2.4.25): apache 2.4.27
PHP version (eg, 5.6): 7.1
Is this the first time you’ve seen this error?: Yes

The issue you are facing: Basically after upgrade all I can see on my server is HTML which appears unformatted. See screenshot:

The output of your Nextcloud log in Admin > Logging:

The output of your config.php file in /path/to/nextcloud (make sure you remove any identifiable information!):

The output of your Apache/nginx/system log in /var/log/____:

One instance is called portfolio and the other nextcloud. It is the nextcloud instance that isn’t currently working

I see a bunch of errors, two being:

[Sun Dec 10 13:05:30.040008 2017] [authz_core:error] [pid 3393] [client] AH01630: client denied by server configuration: /var/www/nextcloud/data/.ocdata

And the second, unable to connect to your database.

Have you recently performed system updates also? Might be worth checking the mysql php driver is still there.