I have been lurking here but most of the advice is very good- especially RELAX and drink lots of water. I would add that if it feels rough, or even hurts, to sing, stop immediately and relax, take some deep breaths, drink some water, and try again. If it still hurts, stop immediately (of course, if you're in the middle of a concert, this may not be advisable!). But pain while singing means something's wrong- either in how you're operating the instrument, or something else.
So, adding to relaxation and water is rest for the tired voice. Sometimes for several days. It sucks, but it's better than permanent damage!