Yes penguin update was been announced by google on 24th april,2012. As per which google tries to remove all spammy sites and by this way it suggests people to make their site structure better to get more results.
And i have also heard that sites which have not natural links, been found their ranking down in search results.
So check on page factors and remove unnatural links to get ranking position again.

its not about penguin update, my site was working well till Monday but changes occur on Tuesday

Wait for few days.. If the case is still the same.. Then PM me with you website link.. I shall see what I can do..