CSC : error CS0006: Metadata file 'file name.dll' could not be found

Hi , when ever i try to make project and run it in visual studio 2012 i get this error :

2>CSC : error CS0006: Metadata file 'E:\PROJECTS\ASP.NET\ProgrammingWiki\ProgrammingWiki.BLayer\bin\Debug\ProgrammingWiki.BLayer.dll' could not be found

what should i do ? i check build is checked in configuration manager in build menu