I seen a spirit or entity what to do?

i no people will think nuts! but last nite i was on my pc then suddenly i felt my bed sink in as if some one had jumped on it!.. i looked up and could see like a greyish transparent cloudish thing! people will think i am crazy but it was like a water ripple in thin air!!! it felt like it went over me my hairs all stood up all over my body i went freezing cold!.. however it didnt harm me i was just a bit freaked out!..