You had an extra closing brace in your code (in the draw loop, the opening brace in an if statement is commented out), which was causing the class not found error.

You will probably want to move all the bounds-checking into your class definition, and let the ball handle it's own collisions (that way you can have a bunch of balls bouncing around and not get tangled up with boundary checking in the draw loop).

Happy coding.

PROBLEM AREA:
//costat dret
//  if (x>=width-w/2){  
   pilota.bouncing(0);
 }


UPDATED CODE:

Ball pilota;

void setup() {
 
 //definim tamany de finestra i color de fons
 size(400, 400);
 background(0);
 smooth();
 strokeWeight(1);
 stroke(255);
 frameRate(30);

 //especificar la situació inicial
 pilota = new Ball(30, 200, 1, 0, 10, 10);

}

void draw(){
 background(0);
 
 pilota.display();  
//movem la posició de x i y
 pilota.update();

}


class Ball {
 float x, y; // The x- and y-coordinates
 float vx, vy; // The x- and y-velocities
 float w; // width
 float h; //height
 float gravity = 0.5;
 float losses = -0.3;
 float resultant;
 float alfa;

 Ball(int xpos, int ypos, float velx, float vely, float wball, float hball) {
   x = xpos;
   y = ypos;
   vx = velx;
   vy = vely;
   w = wball;
   h = hball;
 }

 void update() {
   vy = vy + gravity;
   y += vy;
   x += vx;
   checkBounds();
 }
 
 void checkBounds(){
 //costat dret
 if (x>=width-w/2){  
   vx *= -1;
 }

//costat esquerre
 if (x<=w/2){
   vx *= -1;
 }

//sostre
 if (y<=h/2){
   vy *= -1;
 }

//terra
 if (y>=height-h/2){
   vy *= -1;
 }
 }

 void display() {
   ellipse(x, y, w, h);
 }  
}