How can I get my voice back-FAST??

Vocal rest is imperative; hot lemon and honey will help, and in a worst-case scenario, there's a product called Throat Coat Tea which is fairly miraculous. I lost my voice due to two days of yelling at student stage crews (long story!) and on the morning of the third day I couldn't speak at all. The stage manager gave me a cup of Throat Coat Tea, which I choked down (it doesn't taste nice!!) and within ten minutes I could speak normally. The only thing is that it shouldn't be used regularly as it doesn't cure the problem, just masks the symptoms and if you keep masking the symptoms it can do permanent damage to your voice. However, for a one-off occasion, like your concert, it won't do any harm.