Well first thing I would want to do is nudge it all back into line, if that is possible. If you can't, then you will have to consider pouring a new curb.
If you can line it all up, power wash the crevices and use a bonding agent on the pieces. Then use a cement mix (not a concrete mix, that would be too coarse) to fill the gaps and cracks. I don't have brand names in mind, but I'll re-post if I think of them.