Why did parents abandon chicks?

[goodolpete]

Got a nestbox on my back fence that blue tits started a nest in last month.
Frantic activity since then with nesting material, later with food. Then
last week, nothing. Checked inside box today. Five gorgeous chicks
with blue and yellow plumage, dead, sitting there as if frozen in time.
Why do birds abandon young?

[Nicos]

Maybe they had no choice...ie flew into a window/ killed by a c@t or bird of prey etc etc

Sad isn't it?