What is the reason of a lot of decrement of ranking ?

i have lost my site ranking with main keyword in a month,please tell me.

[mahav]

Recently i have lost my site ranking with major keywords,still i have no change any content at on page, when i do off page optimisation for my major keyword i fail to find that why i have lost my sites ranking with major keyword please give me the suggetion regarding this?

[mahav]

could be the reason of lose ranking with offpage optimisation or offpage optimisation can decrease the major keyword ranking at google tell me immediately?

[mahav]

did a banned directory by google can effect our site ranking if it give him the back link ?