Guys: Reasons why you wouldnt text or ring your girlfriend..?

If you are currently working long hours for about 3 weeks and you are in a long distance relationship with your girlfriend. What would the reasons be for not contacting her? Despite the obvious of being tired, surely if she's on your mind you would text right? Havent heard from him in four days but i have deliberately avoided contact too to see how long it takes him to see if im ok.