Any ideas to why my friends with benefits is ignoring me all of a sudden?

So, I am woman in a open marriage, and four months ago I met a man (on a site for causal relationships) with whom I started a friends with benefits relationship (he is single if that matters). We have always talked very open and honestly with each other and I have been very clear with my wants/need/expectations. I really like him, have a lot of fun both in and outside of the bedroom.
I need to have some connection with the person I sleep with otherwise I can't really feel sexual attraction, that is why friends with benefits works best for me.

We've messaged each other almost daily, not much just always something. He has shared a lot with me during these four months, which for me deepened the friendly aspect of our relationship.

We've talked about ghosting, which we both have experienced and really dislike (who likes it?) and both have said it is nothing we ever have done, or will do. So you can imagine my surprise when I haven heard from him in almost a week, and the four messages I have sent haven't even been read.

Now I know he has been online on the site we first met, which is fine (so am I obviously). It's the sudden ignoring that really gets to me. I have thought about sending him a message on the site, but Im not sure what good it would do since he doesn't even read my regular messages.

My question is, should I wait (it's been like five days since we spoke), should I message him what's going on (if that would result in anything?).

People who have done something like this (ignore/ghost) what is the reason? And is there anything to be done?
Any ideas to why my friends with benefits is ignoring me all of a sudden?
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