Not an answer, but you probably need to start by working out what it's actually composed of. If it's accessible then knock the loose bits off and see what it is. It's already broken, you wouldn't actually be harming it and nobody would consider gluing it back together.
I'm guessing it's concrete. If so then it looks like there's a severe issue with it. You could patch up the missing bits, but I'd guess the rest of it isn't far behind in giving up. I can see another crack further to the left, and what looks like a break through the whole thing on the left edge of your photo.
Is there a slope on the top surface, to shed the rain? If water is gathering and soaking in then that could be the issue.
Also it should have a groove on the underside of its lip, to stop the water tracking under and down the wall, which has resulted in some staining below it.
I reckon you could be looking at getting a scaffold tower, temporarily remove the glass panel and replace the entire sill.
A handyman could make it look nice, for now, with a bit of filler and paint. But I doubt it would last.
Are your neighbours similar, and is it within warranty if newish build?
