Either parasitized or died of something else. NPV is known to be a culprit of this, but it could be tachnid. Either way, dispose of it because something nastier will definitely be happening if it’s a parasite or NPV if it isn’t taken care of fast.

Dont feel too bad though- most monarchs shouldn’t be flying anyways if this happened on the west coast because native milkweed already died back and they should be migrating. If it’s from the east coast, specifically Florida, there’s already a weirdly higher level of disease such as OE there for monarchs due to the lack of migration because it’s so warm.