If your copy of Rock Band for PS2 won't work, try some of these methods to fix it.

[edit] Steps

Clean the disc well.
Run another disc in the PS2 for about 5 minutes, then try the Rock Band disc.
Plug in a standard controller, navigate to the browser screen, and if the PS2 reads the disc, simply select it with the controller.
Insert a copy of Rock Band Track Pack (PS2) and let it run until it gets to the Harmonix screen. Quickly switch this disc for the Rock Band disc, and reset the console.
Insert the disc so that the "K" in Rock Band on is pointed at roughly 6 O'clock. If the disc isn't read, then open your PS2 and rotate the disc one hour to the right. Repeat this until it is working or until it is back at 6 O'clock.

[edit] Tips

Be careful with your Rock Band disc as some of them barely work to start with, and you don't want to damage it more.
Save your receipt because if it doesn't work at all you will want to return it.