I thought that if a variable is defined that way I will be able to access it from anywhere
Unless the variable is both public and static, you can't access it from almost anywhere coz that should be accessed based on a certain instance of an object.
If you wanna access a variable from any window, either make it static or call it based on an instance of that window like
Window1 win = new Window1();
win.Variablex = 1;