as i am using ms office 2007. while inserting a image in ms word, i tried to change the page setup, in the paper tab i changed the
height of the paper to 22', a dialogue appear whether i want to fix or to ignore it. by mistakenly i have press the fix button; and
now every line of that article is on different page, in preview also only the single line is there, how should i get the default
A4 paper setting so that i could get the article on a single page.

It sounds like you have more than a problem with paper size... but to answer your question directly. You change the paper size from the Page Layout tab on the Ribbon. From that tab use the Size button menu and select A4 from it. See screen clip (it's from Word 2013, but the same function applies to Word 2007).