My egyptian mantis is currently L3 molting into L4. He's been stuck in the same posistion for about 18 hours. I'm unsure what to do, here is a picture of what it looks like.
Unfortunately by this time his new exoskeleton has probably already hardened beneath his old exoskeleton. Even if you were able to separate him from his old exoskeleton, he would be disfigured and unable to move and feed on his own.
There's little you can do to help him at this point, and euthanasia might be the only feasible option.
Even if he was escaped from his old skin, his thorax should never be that curved. It can appear to have a slight curve, which should straighten out after escaping his exoskeleton. Next time, I would supply more humidity and give him more water.
Next time you can avoid this by making sure they are hydrated. It's not enough to just be concerned with humidity. I give all my mantids droplets of water to drink from the tip of a bamboo skewer at least every other day. Mismolts have dropped to zero since I started doing this. It's time consuming, but worth it. They actually have grown accustomed to this and anticipate it.