Link to old blog no longer works

Awhile ago I changed my theme and did a full update on one of my websites.
The new theme was put in a different directory than the old one, as well as the content.
On the new blog is a link to the old blog, which used to work. However, recently I discovered that the link doesn’t work anymore.

I could be wrong here, but it sounds to me like you now have ‘https://www.camiguin.ph/blog’ pointed to the new installation and there is no working ‘https://www.camiguin.ph/oldblog’ domain.

I tried finding the content of the old blog (Inferno theme), but I can’t find it anymore.

Try using FTP server access or FileManager at cPanel to look and I would guess your previous installation is still there somewhere. If so, it should be accessible from the outside by using something like ‘https://www.camiguin.ph/(wherever-it-is)’.

I know where the old stuff “should” be, but it isn’t there anymore in my FTP/file manager. As I stated in my message, the old link did work, but it stopped working, after I updated to WordPress 4.2.2.
It’s not in /oldblog. The link to the old blog is in the link in the navigation on the new blog. THAT particular link is not working anymore for some reason.

As I stated in my message, the old link did work, but it stopped working, after I updated to WordPress 4.2.2.

I do not know why or how a WordPress update could/would have deleted a different installation in a different location, but it does sound like your previous site is gone forever unless you had a backup somewhere. In my own case, I have a separate machine where I keep a FileZilla-downloaded copy of everything installed and uploaded at my hosting account “just in case” of something such as this.

As a final check, go to cPanel and phpMyAdmin to see whether your previous database still exists there. If it does, you can make a new WordPress installation in a new folder for your old blog and then edit its wp-config.php to have it use that older database. That will not restore any uploads, but it will recover your content (pages, posts, comments, etc.).

I can access the posts via the dashboard. Can edit them and all that, but when I click view, the permalink doesn’t work anymore.
Everything is there, it just won’t open the pages in the browser.

I can access the posts via the dashboard. Can edit them and all that, but when I click view, the permalink doesn’t work anymore.

I do not understand what kind of overall configuration you have there at the server where it seems you have two different sites — old and new — but the above sounds to me like a problem with .htaccess and I would approach it by deleting the .htaccess for the old site you say you cannot locate anywhere via FTP. So, I am baffled and do not know what to suggest!

??
The old .htaccess was replaced when I switched to the new config.

So here’s the story.

I took over with WordPress installed in the root.
I left everything of the old WordPress in the root, built a new website of which the files are also in the root, and a separate blog which I installed in camiguin.ph/blog.

The new website is running from index.html (and the index.php from the old blog is still in the root).
I made a link from the new blog to the old blog which I called archives. That link used to work.
Then a week or two ago I updated to WP4.2.2. I didn’t check all the links, but yesterday I got a mail from someone who tried to access the archives and got the 404.

So the weird thing to me is that I can’t find the db file with the content of the old blog on the server (it’s supposed to be in the themes folder, right?).
But when I log in to the old dashboard, I can still see the posts, and edit them. But when I click View post or the permalink, it returns the 404 again.

The new website is running from index.html (and the index.php from the old blog is still in the root).
I made a link from the new blog to the old blog which I called archives. That link used to work.

I get lost right there since I do not have any experience with running index.html and index.php side-by-side in a common location. However, I do still suspect this has at least something to do with some kind of change in .htaccess and I do know deleting access and then resetting (re-saving) permalinks can be part of helping WordPress find itself.

So the weird thing to me is that I can’t find the db file with the content of the old blog on the server (it’s supposed to be in the themes folder, right?).

No, the database is on the server at cPanel and visually accessible only from there via phpMyAdmin. The themes folder only contains folders for themes (skins), and that is inside wp-content where you will also find other uploads such as images, audios and the like that are essentially mere attachments for the actual content (pages, posts, comments and such) in your database at the server and accessed by the MySQL user in wp-config.php.

Yep, you were right!
.htaccess was the problem, indeed!
I restored the old version that I deleted, and now it works again.

The only problem I’m now having, is that the redirect to my new 404.html is not working.

I added ErrorDocument 404 /404.html all the way at the end of the htaccess file, but it doesn’t accept it. It goes to the 404 of the old blog’s installation.

the redirect to my new 404.html is not working.

I added ErrorDocument 404 /404.html all the way at the end of the htaccess file, but it doesn’t accept it. It goes to the 404 of the old blog’s installation.

I would guess that also has to do with your .htaccess while having index.html and index.php side-by-side in that same directory, and I would likely deal with that by making/adding a custom 404 document in (or maybe just copy one on over into) the old blog’s directory where your error traffic is already landing rather than complicating your working .htaccess.

Another thought: I have an .html domain/site with only an index page in my root directory and then all my WordPress installations are in sub-directories with their own .htaccess files unique to each site.

Question: Is it your html errors or your WordPress errors that are landing at the old blog? If that is where the WordPress errors are going (as they actually should), try adding .htaccess with you error directives there in the directory for the old blog.