This happened to me quite a while ago. It was still close to summer, or just after daylight savings time, because it was around about 6 in the morning and light was flooding through the windows.
I posted this story on another website, a few weeks after I had this experience, on experience project, so if you find it, it's not stolen, I assure you.
The experience began with a dream. (The dream ties into my experience.)
In the dream I was in a dark, empty room. It had no windows, no doors; no distinguishing features except a fireplace on one side of the room and a fire was on. I went over to the fireplace, and looked into the flickering flames. I noticed there was a white mask floating behind the fire, glaring at me with its empty eyes. Then the fire seemed to "fade" behind the mask, though the fire didn't go out, still glaring at me with its empty eyes.
Then the whole room seemed to fade into blackness, including the fire, which didn't go out; just faded. All that was left was the mask with its eerie white glow and its cold glare. I couldn't breathe; there was something on my chest. It was then I knew the dream was evil. If any of you have sensed an evil presence, you know it is one of the worst feelings you will ever experience. The sheer terror is unfathomable. I forced myself awake, and forced my eyelids open attempting to sit up. Then I saw her.
There was a girl lying on top of me, with dark messy hair, dirty pale skin and pure black eyes with black tearstains down her cheeks. She was holding my chest down, stopping me from breathing. I was completely paralyzed. She made a sort of growl/creak in her throat, and pushed my chest down harder. I think my head lolled to one side because I think I was loosing consciousness... Or worse. Then she faded away. I coughed and spluttered for air.
As soon as I calmed myself down, I went downstairs to hug my cat for some comfort, and watched "The Hoobs" because there wasn't anything else on TV at that time in the morning.
I would like to note, I have not ever experimented with Ouija boards or have ever conducted in Séances, because I know what can happen. I know that no one should ever take part in supernatural communication without knowing what they are doing, or have protection from a medium.