Ive installed the game and played it before, all the way to the end. A few months later, today, i've come back to it and, upon entering the disk, it doesn't do anything at all. I can't find the file to start the game in the installed folder, and the autorun option on the disk does [censored] all. This is why steam is better tbh.

Yeah, well in some cases the steam version is indeed better.
I just bought the retail DVD and although I have no problems with the DVD reading it I did have problems with some in the past. Recently I installed Tomb Raider The Last Revelation (Retail DVD) and it had problems being read having had the game for around 7 years or so. I decided to clean the DVD with a CD cleaner and then it worked!

I recommend you try the same to see if it works. I ordered the DVD of Hunted: The Demon's Forge yesterday, for ￡4.50 from play.com and got it yesterday morning. Enjoying the game so far. Although I too would've preferred it if it was on steam. If this was a steamworks title this wouldn't be a problem even if it was on retail DVD. As it would be registered into steam and you'll have the option to download it from steam like with Skyrim. But this wasn't the case with this game.

But as I suggested, try buying CD Cleaner, they're cheap. Like liquid spray and clean the disk. It should have a chance of fixing it, unless the DVD is badly scratched..