Some stories can sound so utterly terrifying and bizarre that they defy explanation, but there are some clues in the following story from user Lchmst that seem to indicate what happened:
I find myself waking up to the oddest buzzing sound. It was distinct and had a sort of percussive rhythm to it. Kind of like when your ears pop but repetitive so it was very uncomfortable. As I start to wipe the sleep from my eyes I realize I'm sitting up and all the lights are on in the surrounding rooms and the TV has turned to white noise.
I look around the room and start to get a very uneasy feeling that I'm being watched. I shrug it off and start to get up to go take a leak and turn the lights off. As soon as I stand up I feel a cold hand wrap around my throat, it then picks me up and slams me back down on the couch. I start to freak because I don't see anything there and whatever it is must be massive because the hand wrapped around my entire neck, I can feel it's digits meeting at the back of my neck.
So I go to stand up again and once again it wraps it's hand around my throat and pins me down on the couch, holding me there for a few seconds. I start to think, should I fight back or scream or what should I do? Almost as quickly as the thoughts enter my head, I hear a voice saying don't touch it, it will leave if you don't touch it. So I put my hands up, relax myself and the hand let go.
As freaky as this sounds, most of the scariest elements actually add up to some common sleep phenomena, specifically sleep paralysis and lucid dreaming.
The buzzing noise and sensation of being grabbed or held are particularly indicative. Buzzing, sometimes known as Exploding Head Syndrome (which is no less disturbing), is a common observation of lucid dreamers, and the sensation of being held down is a symptom of sleep paralysis, during which the mind wakes but the body doesn't, so it rationalizes its inability to move by imagining restraint.