It depends on how it broke and what it looks like. If you have no idea how it broke, it might be even tougher to fix. But without more information, I can't really solve how to fix it.
What type of information do you need? I'll answer anything I know.
Well, first off, what pieces are broken? If it's his shells that wrap around his brain, and they've only unhinged, then they can easily be snapped back into place. His arms should also snap back. But other pieces, if broken or just fallen off, are not likely fixable.
His upper body separated from his lower body. There are no visible breaks, they're just no longer connected.
If it were me, I'd keep it like that. I think it looks cool. xD
But if I were to fix it, all I can really suggest is try to snap it back into place.
It looks like you need to slide one end in first to make it work.
get some waterweld and sand down the peg, make a new peg with waterweld and it'll stay
Uhhh.... give it to Bandai so they can fix it?
Looks like it could snap back on. But I'd go with LA (Feedback) and try taking apart the top half to attach it back on.