Mama Mirabelle, it sounds like something has suddenly made her scared about "school" so she has regressed so she doesn't have to go.
Maybe try some positive reinforcement. Tell her how much fun school is, how they do drawing, painting, singing etc (anything she is interested in!) and how she will make friends and have lots of fun! Could you take her for a visit so she can see it's nothing to be scared of? Try chatting to her as well, if you gently ask her why she doesn't want to use the toilet, and ask if she's sad/worried about school, she may tell you if you can get the right question for what she's feeling.
I agree with bribery usually, but I think this is more mental than physical. And I'm sure you know this, but your DH's reactions won't be helping.