What should I do about a player who isn’t engaging?

So recently the party I DM for has had to move to playing online (due to covid and social distancing) it started off fine and everyone was having a good time but over the last two sessions one of the players is just refusing to get involved at all.

They will come to the session but then just sit in silence for the entire session.

For example their character was woken up to take watch for a long rest and they just refused to do it, the following session the party got into combat and when asking what everyone rolled for initiative they said they were just going to sit in the cart while everyone else dealt with it.

As the DM I don’t see the point of them attending sessions anymore since they aren’t doing anything but I don’t know how to approach that conversation, any ideas?