What should I do if I lose my voice before I have to sing?

Rest your voice to keep your vocal cords from getting aggravated. You can also do some kind of cool down exercise, like lip trills or hums, after each singing session to keep your voice from wearing out. Sleeping with a humidifier on is also a great way to keep your voice from drying out at night. You should also drink plenty of water to help hydrate your throat. If this is a consistent problem, consider reaching out to a vocal coach. There are a lot of laryngeal manipulation exercises you can do to improve the strength and health of your voice.