You used to have x, but now you don't anymore. You cleared it somehow, like by calling the clear() or delete() function. For example, if you have any lines like these inside your function, remove them:
clear all; % Remove this line from inside any function.
clearvars; % Remove this line from inside any function.
clear('x'); % If you have this in your function, remove it.
Can't say much more without seeing your code.
