Hi, sorry to hear your piggie isn't well. If your piggie has stopped eating, you need to step in with syringe feeding right away. Either critical care or pellets in warm water, mashed up and allowed to cool.
If she is completely off hay, it's important to try and feed her at least 40-50 ml of syringe food everyday, maybe around 5ml every 3 hours. Obviously make time for your rest time too, as looking after a sick piggie is exhausting.
Syringe feed her water too when she will take it. She will get water in the syringe food so do not force feed her too much water. Please start out with just a small amount of feed/water like 0.3ml at first just to see if she is able to swallow and to avoid choking.
You can also try adding some probiotics to her syringe food to try get her apetite up and keep her guts going. Probiotics do a good job at keeping the gut happy and stable.
Piggies can go downhill very quickly, so one day they may be fine and the next not good at all. With them being prey animals, they are very good at hiding their illnesses until they cannot any longer. Unfortunately by this time their illness may be rather advanced. If you are concerned I would definitely have her seen by a vet. I hope your piggie is OK.