Alright so i am a proud mac owner and love it. Well my friends and I used to record our own music and download it onto my itunes library by using garage band. It was soooo cool we like made 3 whole albums on it. But then randomly one day I went to record music again with my friends and you the little icon thing on the dock is where I click to get it to open. Well it didn't open and in front of the icon, which is a little guitar, it has question mark over it. Why is it doing this and how can I fix it?!?!?!?!?!?! Computer, repair.

ok well my boss what counting on me for this. i started to burn some songs off of itunes and so far i only have like about 5 in there. then i took the CD out. later i came back to the computer to burn more but it said "this is not an empty disk" plz plz plz help. and no, gettin another CD is out of the question. i want to use the same exact disk Computer, repair.