As I recall, some Wiis are shipped unable to read dual layer games (i.e. Brawl). This means that you either have to open it up to increase the lens sensitivity via screw (which will in turn shorten the lifespan of the lens) or send it to ninty to do it for you.
Around the time Brawl came out, ninty also released an announcement saying that dirty lenses (most often if someone smokes a lot in the room the Wii is in) can also make it not work anymore. Sending the Wii to ninty will let them clean it for you for free (and/or increase the sensitivity as above). The last point is that the disk might also be dirty, in which the solution is obvious.
All that assumes that this is a retail game (it might just be me, but you didn't make it too clear). If it's a backup.... solutions are many - check image, media, burning program, cIOS, and Backup Loader to make sure it's all good. I've yet to see a backup version of Brawl run subspace emissary without crashing at some point, but I know it works in general.