Meaning of ならなくなった in this sentence

Question

From Shirabe Jisho's example sentence

係長が急に辞めてしまったので、私がその穴を埋めなければならなくなった。

While I understand that 穴を埋めなければ means "if we do not fill the hole" - so the conditional form of the verb - I cannot seem to understand what this ならなくなった is being attached to. My guess would be that it is some form of the verb なる ("to become").

Answer 1

It's a combination of ～なければならない and ～なった.

• ～なければならない essentially means "must". See: Origin of ~なければ ならない
• ～なった at the end indicates the progression of a situation. "became", "ended up with a situation where ～", "found myself", etc. It takes the continuous form (ku-form) of ない.

Or you can understand the sentence incrementally like this:

• 埋めなければ
  If I don't fill
• 埋めなければならない
  If I don't fill, it's no good
  → It's necessary to fill
• 埋めなければならなくなった
  I ended up with a situation where it's no good if I don't fill
  → It became necessary to fill