Basically, we call it either project references are code lens feature in visual studio, some times it stops working. in this case, just stop and start code lens feature.
To do this just go to
Tools -> Options -> Text Editor -> All Languages -> CodeLens.
Uncheck \”Enable CodeLens\” option and click OK.
Go back into Tools -> Options ->Text Editor -> All Languages -> CodeLens.
Check the \”Enable CodeLens\” option and click OK.
/Siva