Refusing to eat, drink, bathe or get out of bed!

That is a challenge, no doubt.

Is she depressed? Has she been evaluated recently?

Have you tried to calmly ask her why she won't do anything? Has she just given up and wants to die? How advanced is her dementia?

Your Mom may be signaling that she personally is "over this" and that she is ready to go. This is not untypical near the end. You may now want to investigate hospice care or palliative care for her. She should have a complete physical at this time, and be certain a urinalysis is done if this is new or sudden change for her. She may be having some vascular changes, mini-strokes or what have you that are being missed. You do not mention her age. End of life is full of losses. Not everyone wants to see it through for a long time. Does she speak with you at all about her reasons, or has she simply reached the "turning the face to the wall" point. I think if she is in Assisted Living they will not long be able to maintain her in that level of care. I am so sorry. This is hard to see, and hard to imagine feeling, but it does happen to many of our elders.

How long has she been refusing to get out of bed?
Do you or does anyone have any idea why?

This could be something as understandable and treatable as acute depression. How responsive is your mother when you speak to her? Has she been examined by a doctor?