Do various warm ups everyday, but don't strain your voice by trying to sing to high or low right away. You need to increase your range VERY gradually. If you don't currently have a voice teacher try to get one. Voice lessons have really helped me. Or ask your choir director for some good advice. Hope I helped.

join da club. that always happens to me! but sometimes if i either drink a lot of water or take a deep breathe, it might work. so try it.

That does happen to a lot of people (including me). What I do, is I drink a few sips of water, take a deep breath and do various warm-ups, such as humming like you would if you were meditating, or doing the musical scale a few times through. If it still doesn't work then, just drink a little more water and take your mind off of it. Wait about 2-3 hours before you sing again. Hope this helps!