I have been there, Gwynne.Donna is right.. all you can do is watch and wait.
When we first startied rearing Monarchs at school we would hover over them and try to get them to eat. All the literature makes it seem that they should never stop eating. But they do take time out to rest and moult. As long as you are providing fresh leaves and they are healthy caterpillars.. they will know what to do.Three days of no eating however, does seem like a long time. I have had batches that have just stopped eating and then died. There are lots of bacterial,viral and protozoan diseases that can kill monarch caterpillars.
Sad but true.
I am playing a waiting game with mine too....My Ac went on the fritz while we were out and the milkweed that the newly hatched cats were on went funky.I cut the pieces off and placed them on fresh leaves ASAP...So we will see.
And Perhaps someone knows about this symptom...
One of my older cats ,just prior to pupating, expelled a drop of brownish/red fuild.That can't be good, huh?
I guess all we can do is our best!
It's all part of raising Monarchs.