are u missing an assembly or referrence ;

I have not modified anything at code but still getting the same error frequently;;
I use VS 2008, SQL 2008 , SOA

Error    2    The type or namespace name 'UserMaster' does not exist in the namespace 'HealthCare.UserService'
(are you missing an assembly reference?)    HealthCare\Pages\SuperAdmin\UserAdministration.aspx.cs