What makes a horror novel or story "scary" for you?
Jump scares? Nope. Sorry. The reader controls the pacing. And he or she can skip ahead or backward at will. That clutching crone hand can go backward and forward and be skipped altogether based on the reader's whims.
Graphic visual scares (or gore)? Sorry again. Unless you're most visceral writer ever, written gore falls short.
So, as a writer you're stuck with having to be a psychological and writing genius. But how? While I'm far from an expert on horror, I have written several tales in the genre, and I've learned a few things with each successful telling.
1. Be visceral. But don't mistake visceral for gross. For example, while a limb being removed and force fed to a tied up victim is certainly a compelling image in a story, it may not be as effective as something as simple as a sewing needle being wedged into the soft skin beneath a dry fingernail.
2. Tap into the universal fears. For example, when I wrote "Nymph" for the Gene Simmons House of Horror graphic novel collection (yes, I know that it's not pure prose, but bear with me), I wanted to recreate the sense of being lost in the woods, in a place where you're at the mercy of the natural world. When I was a kid the woods were creepy sometimes, and I had lost that feeling after moving to Atlanta and growing up.
3. Discover the specific, individual fears that make a person tick. For example, in my zombie tale "Posthumous" (from Zombiesque by Daw/Penquin Books), it's not the decaying body of the zombie that makes her creepy. It's her determination to save her marriage, her blind, unwavering determination to do so regardless of the consequences to anyone else.
4. Unleash your horrors on ALL the senses. Don't let just sounds and sights convey your protagonist's woes and horror. Go deeper. Is that smell like the burn ward at a hospital? Does the touch of the killer leave grease and sweat on a victim's neck? Does the hooker's kiss taste like she's been eating rotting meat? Engage all the senses that can convey fear and discomfort.
5. Use sounds that bother the reader, not just the characters. You can make up words that sound like stuff. The official literary term for this is onomatopoeia, and it works because it plays games with the reader's ear, whether they hear the sounds spoken aloud or not. For example, in my steampunk horror tale "Death with a Glint of Bronze" for Dreams of Steam II: Brass and Bolts, I hit the reader right of the bat with the "crick-cracking of the neck bone where it attaches to the top of the spine." But the following sentence continues the idea, simply by using sounds that create a stop and reflow, like restricted breathing might sound: "Then there is the delicious constriction as the breath slowly ceases its movement through the windpipe."
6. Don't try to be "horror movie" scary. Aim for "imagination" scary. Go for the stuff that no movie could ever film, you know, the kind of sick, warped, crazy stuff that could only take shape in someone's imagination as they read. For example, does anyone really know by reading Lovecraft's stories what an elder god truly looks like? We have ideas, but that's all. We have the accepted image that has become synonymous with the tales, but be honest -- does that fully match the horror you imagined in your psyche when you first read the words of HPL's description? On a similar note, isn't your personal nightmare of Lewis Carrol's Jabberwocky far creepier than any of the drawings you've seen of it?
That's all I've got to give you, but if you can even those six things well, you'll never hurt for a job writing truly frightening horror tales.