Um, not very specific...
What version is your wii? 3.4? 3.3? 3.2?(recommended) or 3.1?
Do you have Softmii installed?
Is your Wii NTSC-JAP, NTSC-USA, or PAL?
Are you using the Backup Launcher 0.3 Gamma?
Are you using the Backup Launcher 0.3 Beta?
Are you forcing PAL or not?
Are you trying to boot it directly from the disk channel?
Have you burned successful disks before?
Are you using an original or burned copy?
Do you have a modchip installed?