This is what I did to fix it.

First I found this support thread: http://wordpress.org/support/topic/334199

but although showing someone else having the same problem, it didn’t give a reason why or offer a solution!

Then I found this, older, support thread: http://wordpress.org/support/topic/224950 this time with the statement “I fixed it!” and showing how it was fixed…

Brilliant, I thought, all I need to do is to make sure my htaccess file included the code shown in the “fixed it” post (shown below)

So, happy with my detective work, I accessed my htaccess file…only to find that it already contained the exact code required (pictured below)

?????

But I decided to copy and paste the recommended code over the existing code and re-upload the file anyway…

I reloaded my blog and all internal links we’re no longer leading to 404 error pages!!

Everything was back to normal!

It’s great that everything is working as it should again and there’s been no further problems with links going to 404 error pages since.

The only thing is that it leaves the question why it happened unanswered.

I hadn’t made any changes or updates during the time between when all internal links were working and when they all stopped working!

If anyone has any ideas it would be great to hear from you!