I have everything ready to go to write a story, yet when I sit down to type it, my motivation leaves. help!?

I love writing stories, my favorite is mainly horror, in that genre, my favorite is slasher. few friends find themselves victim of a masked killer and at the end you discover who its is. mystery, but boring. but my problem is i cant seen to write a story anymore. i have ALL characters planned out, plot thought out great, the twist ending running in my head and excited about how surprising it will be to a reader, yet when i sit in front of the computer to type it, its as if my motiviation leaves. Am I trying to do something that I really dont want to do? my main motivation is my horoscope, id read it for that day and then try to think of a good story, but i always think in the horror genre. should i work with others, drama thriller etc. i was once told to use major life moments as a story idea, yet once again i thought in the horror genre and no ideas. what am i doing wrong help!