I mined on a few window pc. but after 24 hours. hashrate in my moniter pool was reduced by about 50%. And my earning receive reduced. anyone know why?

Maybe more miners, bigger difficulty, less coins ?

The difficulty has doubled since the mention of the cloud mining hack
Those with a single cpu will not get anything in comparison to the massive hash power from cloud miners
You are exactly right with the higher difficulty the small guy gets less and less