Please Help

I have a problem with uploading pics in 2.7
I can upload it and also gives me all the options in the uploaded window like it should normally work… when its done uploading the picture doesnt show on the right upper side of the gallery as it normally should. It looks blank. i have tried all the options : including it in the post or just as a thumbnail or posting the link but it doesnt show like other pics in my gallery.
in other ords i upload the picture, attach it in my post and i cant see it anywhere. in the homepage the thumbnail of the pic attached to the article shows the red X and a blank box instead of the picture.

Ps : i have other pics that i have uploaded long time ago and i am enamble to see them either in the post or in the gallery from my admin panel, the thing is that it has been a wekk that i cant upload any pics. I have tried jpeg. if png nothing works. i re installed my windows coz i thought it was related to a program i installed and it still doesnt work. so its definitely wordpress but i dont know how to fix it.
any idea?? by the way the permition of uploads file in the wp-content are 755