married for 6 years no physical relationship past 3 years

Married for 6 years no physical relationship 
Me and my husband have been married for 6 years now we use to be very active for the first 3 years. Past 3 years and there has been no physical relationship. I'm not sure what it is lost of interest or what? He says he is too tired. I don't know what to do? This is causing fights in our marriage?