Should have just let it sprout, not messed with it. The cotyledon will not come back, you ripped it in half it will continue to grow this way. This seedling not off to a great start, generally speaking first few weeks of a plants life most important, hard to raise a healthy plant from an unhealthy seedling. Personally I would germ another seed if you have it, or continue with this one it may just struggle a bit and come good. Just dont overwater, and no nutrient yet. Best of luck.