Validation of viewstate MAC failed OR Invalid viewstate

Hi, I'm getting this error in almost all of my pages.  What is the reason.?  How to handle it.?  From last few days I've many users on my site nd from that time I'm getting this error.
PLESK-WEB14 Error in:http://www.sitename.com/mainpage.aspx Error Message: Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster. at System.Web.UI.ViewStateException.ThrowError(Exception inner, String persistedState, String errorPageMessage, Boolean macValidationError) at System.Web.UI.ViewStateException.ThrowMacValidationError(Exception inner, String persistedState) at System.Web.UI.ObjectStateFormatter.Deserialize(String inputString) at System.Web.UI.ObjectStateFormatter.System.Web.UI.IStateFormatter.Deserialize(String serializedState) at System.Web.UI.Util.DeserializeWithAssert(IStateForma ...