most likely the laser is dirty, damaged or even broke. try cleaning it by wetting the tip of a que tip and putting a little soap on the quetip and gently rub it on the lesn, then rub it off with the dry side of the que tip. let me know if it works ok??
this means your gamecubes laser burned out and you will have to send it into nintendo this is a common problem facing gamecubes as they get older for some reason or another the laser gets worse
I have a GC that does that, but I don't think it's the laser, it's that the disk isn't spinning. so check to see if the disk is spinning or not. because if it's not, then theres no point in replacing the laser lens. Not sure what to do to fix it.
