Hey Mohit,
We'd love to help out, but we need more info.
- Did you recently switch to Magento or is this problem irrespective of Magento?
- Were there any big changes on the site?
- When did you notice the drop in rankings?
- Do you know which iteration(s) of Panda and/or Penguin you were affect by?
Let us know and we'll be happy to help!
-Mike