beachutter

Wonder if anyone can help. I have recorded an arrangement in Logic, and it was fine to begin with, but I am not now getting any sound from certain recorded tracks (Logic software instruments). I have Logic Pro linked to my Clavinova keyboard via M-Audio. It has worked OK until now, but I can't hear any sound from my monitor speakers even though an audio signal is appearing in the channel strip. Neither am I able to hear a Logic Intrument when I play some notes on the keyboard. Any help would be appreciated. I am new to this forum.