Without seeing any code, I can only guess...
But I got this error (again!) yesterday when I wanted to add a function to a sketch without setup() definition.
In other words, you start coding a little sketch, a few lines running fine without setup nor draw. Then you think it would be nice to save it, so you add your function. And bam! it doesn't compile anymore.
As said, add setup() (perhaps with noLoop()), and draw() (I think it is necessary to take in account keyboard/mouse events, can be empty).
Put your code in setup() or split it between init code in setup() and draw code in draw(). In the later case, don't forget to put declarations of variables shared between both functions at sketch level.