Question about my SO

We used to have sex probably 4-5 times a week, for a solid 2 years. The past 3 weeks, we haven't had sex at all. He won't even touch me, like hug me, cuddle with me when we go to bed, hold my hand, kiss me, etc.
Literally nothing has changed in the relationship, no fights, we both just go to work and come home, I cook dinner and do some cleaning, he browses the web, and watches tv. Everything is literally the same in our relationship as it has been for the past two years, except he rufuses to be any kind of intimate with me.
Does anybody know of what could cause this? Does anybody have any experience with this?