Don't worry, your Neopet will be okay. There are several different ways to heal them. One way is feeding them. If you simply feed your Neopet, then it will regain some health. If you don't have time to wait, there are always Healing Potions you can feed your Neopet that will speed up the healing process. Another option is to go to the Healing Springs in Faerieland. There, a nice faerie will look at your Neopet and help you.