Hello, I had a look at your code and have re-ordered the if statements, it was also getting caught if you went to the far left of the screen too.  I also made the hero a bit faster so I could win. ;)

//_____________________________Ce void gère l'affichage et le mouvement du héros _______________________________________________________________________________
void Heros() {
 
  image (BH, x, y);// fait appel à l'image du héros et l'affiche aux coordonnées x et y prédéfinies
  if (p==0) {//ici la condition p=0 indique que le jeu est en marche (voir void pause)
    if (keyPressed) {
      if (key== 's'||key== 'S') {
         if (y<425) {
              y=y+10;
          }
      } else if (key== 'z'||key== 'Z') {
         if (y>75) {
           y=y-10;
         }
      } else if (key == 'd'|| key == 'D') { //mouvement horizontal vers la droite
          if (x<950) {
            x=x+10;
          }
      } else if (key== 'Q' || key == 'q') { //mouvement horizontal vers la gauchedddd          
          if (x>50) {
            x=x-10;
          }
        }
      }
  }
  else {//cette condition est le retour de la condition de mise en marche du jeu. Si le jeu a été mis en pause, les variables x et y restent fixes (voir void pause)
    x=x;
    y=y;
  }
}

Thank you verry much this helped me out a lot, could you try to explain me why my code didn't work, i do understand the changes you made but still don't get what was so wrong about my code...

It was because you were checking to see if x>75 everytime the key was pressed so when it hit the top it would never be true.