I used to be an A student and really enjoyed school......but now......?

now i cant stand it, i get distracted too easily, i fall asleep in cl, i got my first F, and got five Fs in my interim........i've lost motivation and i've become so lazy.....its my junior year in high school and the most important........im so frustrated and whenever someone asks me wats wrong i burst into tears.......wat do u think?? is it something psychological or just plain dumbness.........