This is the 9451st most frequent Japanese word.
立た
"立た" is the stem form of the verb "立つ," meaning "to stand" or "to rise."
In this sentence, '立たない' is the negative form of '立つ,' meaning 'to stand' or 'to rise up.' It indicates that the children did not stand up.
子供たちは立たないので、先生が注意しました。
The children didn't stand up, so the teacher gave a commentary.
Here '立たざるをえない' conveys a nuance of compulsion, literally, 'had to stand,' denoting the necessity of standing at a specific place.
その場所で立たざるをえなかった。
At that place, I had no choice but to stand.
In this sentence, '立たなくなる' indicates a transition, specifically 'to become unable to stand,' metaphorically referring to the machine ceasing to function or operate.