Have you ever sing on stage and forget the lyrics? what will you do?

Anybody who has to memorize lines has that problem. Suggest you take a tip from actors, and use cues to help you remember the lines. Musical cues can be found in most arrangements, like bars, particular notes, an instrument at a point in the song. This isn't from experience, I'm a musician, not a singer, but I thought it might help.

I am a singer and I never perform from any sheet or piece of paper with my notes on it - I indeed don't perform until I am confident with the song. I simply practice until I know it so that I don't have to think about it - just as a child would learn a new word or even a song: they repeat it over and over again, until they can use it without mistakes. I don't think there is any better way then that. Because if you use some kind of clues (like "now I count to four here, and here, after the 9th beat comes the most difficult part which is..."), then when performing, you have to concentrate on these clues. And what happens when I do forget some words of a song on stage? Nothing. I would sing any words that comes onto my tongue - it's anyway 2 or 3 seconds maximum and it's fine again. I wouldn't get stuck on it, but make sure that the rest of the song is great.