A rede falhou; daí o apagão.

Here rede means network or, more specifically in this context, the power grid / electrical network.

On its own, rede can mean several things depending on context, for example:

• rede elétrica = electrical grid
• rede de internet = internet network
• rede móvel = mobile network

So in A rede falhou, the exact meaning comes from the situation. If the sentence is about electricity, rede is best understood as the grid.

Falhou is the pretérito perfeito simples of falhar.

• falhar = to fail
• falhou = failed

So A rede falhou means The grid failed.

This tense is used for a completed event in the past. It presents the failure as a single finished occurrence, which fits well here because it is the cause of what happened next.

Compare:

• A rede falhou. = The grid failed.
• A rede falhava. = The grid was failing / used to fail.

The sentence wants the first meaning: one specific failure.

Here daí means hence, therefore, from that, or as a result.

So:

• A rede falhou; daí o apagão.
• The grid failed; hence the blackout.

This use of daí is a concise way to show result.

Be careful: daí does not always mean this. It can also literally mean from there in other contexts.

For example:

• Saí de Lisboa e daí fui ao Porto. = I left Lisbon and from there I went to Porto.

But in your sentence, it is a logical connector, not a location word.

Not exactly, though they can be similar.

In this sentence:

• daí = hence / resulting from that
• por isso = therefore / for that reason
• então = then / so

All can connect cause and result, but the tone is different.

• A rede falhou; daí o apagão.
  Very compact, slightly formal or journalistic.

• A rede falhou; por isso houve um apagão.
  More explicit and neutral.

• A rede falhou, então houve um apagão.
  Very common, more conversational.

So daí o apagão sounds tighter and more elliptical than the other versions.

The semicolon links two closely connected ideas:

• cause: A rede falhou
• result: daí o apagão

It is stronger than a comma and tighter than a full stop.

So the semicolon helps show: these are separate parts, but they are very closely related.

You could also write:

• A rede falhou. Daí o apagão.
  Also possible, slightly more abrupt.

A comma would usually be less good here in careful writing because daí o apagão is not just a small aside; it is a separate result clause.

Portuguese often uses the definite article where English may not.

So o apagão means the blackout.

After daí, Portuguese commonly uses this pattern:

• daí o problema = hence the problem
• daí a confusão = hence the confusion
• daí o atraso = hence the delay

So daí o apagão is a very natural structure.

Without the article, daí apagão would sound wrong.

Something is omitted. This is an example of ellipsis.

The fuller idea would be something like:

• A rede falhou; daí resultou o apagão.
• A rede falhou; daí veio o apagão.
• A rede falhou; daí surgiu o apagão.

But Portuguese often leaves that verb out when the meaning is obvious.

So daí o apagão is a compact way of saying:

• hence the blackout
• which led to the blackout
• resulting in the blackout

This kind of shortening is common in written Portuguese, especially in journalism or formal summaries.

Yes. It sounds natural, especially in written, journalistic, or concise explanatory language.

It is less like everyday casual speech and more like something you might read in:

• a news report
• an analysis
• a formal summary
• a technical explanation

In everyday conversation, a speaker might more often say:

• A rede falhou, por isso houve um apagão.
• A rede falhou e depois houve um apagão.
• Como a rede falhou, houve um apagão.

So the original sentence is natural, but stylistically a bit compact and polished.

In European Portuguese, falhou is pronounced roughly like fa-LHYOH, though the exact sound is not easy to represent in English.

A few points:

• lh is a special Portuguese sound, similar to the lli in some pronunciations of million, but not exactly the same.
• The stress falls on -lhou.

So:

• falhou = fa-LHOU

If helpful, think of it as one syllable after fa:

• fa-lhou

The lh sound is important; do not pronounce it like plain English l + h.

Apagão usually means a blackout, especially a major power outage.

It often suggests something bigger or more serious than a small local interruption.

Compare:

• corte de eletricidade = power cut / electricity cut
• falha de energia = power failure
• apagão = blackout, often large-scale or dramatic

So in this sentence, apagão is a strong, natural word if the outage was significant.

Not naturally in English.

A very literal breakdown might tempt you toward from there, because daí can literally mean from there in other contexts. But here that would miss the real meaning.

In this sentence, daí expresses consequence, not physical movement.

Natural English translations would be:

• The grid failed; hence the blackout.
• The grid failed; therefore there was a blackout.
• The grid failed, causing the blackout.

So it is better to translate the function of daí, not just its most basic dictionary meaning.