It's possible that it's getting ready to molt.
A praying mantis won’t eat a few days before it will shed its skin (molt). This is normal. After molting it will start to eat again. When a praying mantis will not eat even though it does not need to molt, it can help to offer it a different prey species. Do not worry too much, a mantis can live for 2 weeks without any food.
A few days before molting, the mantis will hang upside down and will not eat. Sometimes a white film-like layer can be seen all over the body of the mantis. This is the skin becoming more loose. When the mantis is becoming subadult or becoming adult, you can see that the wing buds (location where the wings grow when the mantis is still a nymph) are swollen.
Not sure if the black lines are part of it, but since you said that it is not eating, and from the picture it is hanging upside down, I would say its beginning it molting cycle. As that website states, if you don't think it's getting ready to molt, try switching what it eats after a few days. It seems they can go quite a while without any food.