Due to woeful planning on my part and incompetence on our (former) SSL provider's part, our SSL certificate expired on Wednesday. Because we enforced Strict Transport (HSTS), this meant that no-one was able to visit the site (if they used a browser that supports HSTS).
To keep a long story short, I disabled HSTS yesterday to provide some level of access to the site. Now, we've switched to using certificates generated by Let's Encrypt, a free certificate authority, and re-enabled HSTS. This means that site access should now be restored and we should be back to having sweet, sweet SSL goodness.
Again, I want to offer apologies that this happened in the first place, we should have been better prepared for this scenario.