I have the website which is displaying the following error.

Check this: http://blogs.msdn.com/b/tom/archive/2008/03/14/validation-of-viewstate-mac-failed-error.aspx

Please, check this:
http://stackoverflow.com/questions/6259844/asp-net-validation-of-viewstate-mac-failed

Hope that helps.

I find it hard to believe this suddenly happened.  Someone changed something.  If you don't know who, you should find out.  The fix is straightforward enough:
     http://forums.asp.net/t/1166634.aspx

The Web.config contents are as follows: Where do i put the code so that the error goes off.

<?xml version="1.0"?>
<configuration>
 
      <connectionStrings>
            <add name="sSqlConn" connectionString="Data Source=localhost;Initial Catalog=incnew1;uid=xxx;pwd=yyyy" providerName="System.Data.SqlClient"/>
      
      </connectionStrings>
      <system.web>
    <customErrors mode="Off" ></customErrors>
            <compilation debug="true"/></system.web></configuration>

within <system.web></system.web> elements

like this: Server is not in my control, i only have ftp access to it. Do i need to get the IIS restarted.

<?xml version="1.0"?>
<configuration>
 
      <connectionStrings>
            
            <add name="sSqlConn" connectionString="Data Source=localhost;Initial Catalog=xxxx;uid=yyyyy;pwd=zzzz" providerName="System.Data.SqlClient"/>
      </connectionStrings>
      <system.web>
    <customErrors mode="Off" ></customErrors>
    <machineKey validationKey='79DBE6362149214234C1E3C0C88331F977689D1A0B3DCA05530AE6A8E7F5DDB3FB9A87069D25F0970C70828491FAF1F8837E1F9A9220B6D4E1E5BEFC9A99E6E4'   decryptionKey='B18DA68275F82D4228C5BB8A491FED929A13C28527A7EF19'   validation='SHA1'/>
            <compilation debug="true"/></system.web></configuration>