'Roles' does not contain a definition for 'GetUsersInRole'

 Dear All,
 
I do not know why it happend to my website suddenly!   I did not have any error now it keeps get errors and do not know any definition for Roles!!!! something like this:
 
 if (Roles.IsUserInRole(theUser, "x"))
 
there is blue underline under 'isUserInRole' ??!!!! 
 Thanks for your helps in advance,
 
Regards,
 
Salma

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