BLUE TONGUE SKINKS

I was wondering if it was possible for a bluey to "drown" in substrate. I moved the substrate around a bit when he was under there, and he hasn't moved for about 24 hours.

No, it really isn't possible for them to "drown" in normal substrate.

Sometimes don't see mine for days in winter.

Wait, what?? Last I heard you didn't have a bluey yet but were setting up an enclosure to be ready. You have been holding out on us!! Feed us with story and pictures

If you have a new bluey, especially if young, it is not unusual for them to hide away from you for the first week or so until they get used to their new home. They do like to bury themselves under the substrate, that's a big reason why it is added to the enclosure to help them feel secure and give them something to dig around in to find a nice spot. Make sure all the temps etc are right. But do need to make sure they are healthy, so story and pics would help. It is recommended that if you are sure they are healthy and the enclosure is right that you don't dig them out unless there's a good reason as it will make them feel insecure and exposed. Also lay off handling perhaps until they have had time to settle in.

Here is a good thread on first few weeks from the FAQs