hi .

i have a website. is dropped from 1 st page of google to last page . What should i do now and how can my site back in google .

First you've got to work out why you dropped, then you need to address it.

Questions>
What have you done to your site recently or immediately prior to the drop?
-make a list, share it here

Have all pages that previously 'ranked' page one dropped, if not which ones?
- describe the sort of pages which dropped, are they content pages, directory, products, news?

Are you running the sort of website that may have fallen foul of Google's Panda update?
- this means spammy, duplicated content, low user value website or a website relying on such websites for links. Also some websites that relied on long tail keywords for organic traffic or websites that have a specific keyword in the domain name.

Why has it dropped, can you explain what type of website you have, business sector, and what SEO has been done?

well my site is also jumped from second page to 10th page.just waiting to come back on same position.