VS 2012 - Suddenly getting many errors when I build my solution

I have a web site that has worked fine for several months.  But now when I build / rebuild my solution, I get tons of errors.  Not sure why this suddenly started.  The errors causing the problems (I think) are like this:

Error2Metadata file 'C:\TFS\V.2.6.5-VS2012\Src\XXXSoftBL\XXXSoftBL\bin\Debug\XXXSoftBL.dll' could not be found

XXXSoftBL is another project in my solution.  It contains most of the business logic.  I have several errors like this.  Why would I suddenly start getting these errors?  How can I fix this?