Learn Hardcore Portuguese (Brazil): Meu celular quebrou de novo, e agora o botão de ligar não funciona. - My cellphone broke again, and now the power button doesn’t work.

Because celular is masculine in Portuguese (o celular), so the possessive has to agree in gender: meu (masc.) vs minha (fem.).
Examples: meu celular, but minha bateria.

Could I also say o meu celular quebrou de novo?

Yes. Adding the article (o) is common and natural in Brazilian Portuguese, especially in speech: o meu celular, a minha casa.
Using no article (meu celular) is also correct and sounds slightly more direct.

Why is quebrou used here (preterite)? Why not quebrava?

Quebrou (preterite) presents the breaking as a completed event: it happened (again).
Quebrava (imperfect) would describe an ongoing/repeated situation in the past or background context (e.g., Meu celular quebrava toda hora quando eu era criança), which doesn’t fit as well for a single “it just broke” event.

Does quebrou mean it literally broke into pieces, or can it mean it stopped working?

It can mean both, depending on context. In everyday Brazilian Portuguese, quebrar is very commonly used for electronics meaning to break / to stop working (not necessarily physically shattered).
If you want to be extra explicit, you might also hear parou de funcionar (it stopped working).

What exactly does de novo mean, and can it go elsewhere in the sentence?

De novo means again. It’s very common and natural.
Placement is flexible, but your sentence is the most typical: Meu celular quebrou de novo. You could also say De novo meu celular quebrou, which emphasizes the “again.”

Why is there a comma before e agora?

The comma separates two related parts: 1) Meu celular quebrou de novo (what happened)
2) e agora o botão de ligar não funciona (the current consequence)
It’s not strictly required, but it helps readability and matches natural pacing.

What does o botão de ligar literally mean? Why use de?

Literally: the button of turning on → “the power/on button.”
Portuguese often uses noun + de + infinitive to describe a button/function:

botão de ligar (turn on)
botão de desligar (turn off)
botão de aumentar o volume (turn up the volume)

Could it be botão para ligar instead of botão de ligar?

Yes, botão para ligar is understandable and grammatical (more like “button to turn on”), but botão de ligar is the most idiomatic/common phrasing.

Does ligar here mean to call or to turn on?

Here it means to turn on. Ligar can mean both to call and to turn on, and context decides:

Vou ligar para você = I’ll call you.
Vou ligar o celular = I’ll turn the phone on.

Why is the second part in the present tense: não funciona?

Because it describes the situation now: the phone broke (past event), and now the button doesn’t work (current state).
Portuguese commonly mixes past + present like this when the result is still true.

Do I need o in o botão de ligar, or can I say e agora botão de ligar não funciona?

You normally need an article or some determiner in front of botão. E agora botão... sounds incomplete.
You can say:

e agora o botão de ligar não funciona (most natural)
e agora esse botão de ligar não funciona (this button…)

Is botão de ligar the only way to say “power button”?

It’s a very common way, but you’ll also hear:

botão de ligar/desligar (on/off button)
botão power (common in tech talk)
botão de energia (less universal, but possible)

Any pronunciation points I should watch in this sentence?

A few common ones for English speakers:

quebrou: the qu sounds like k (keh-BROU).
celular: stress on the last syllable: seh-lu-LAR.
botão: nasal -ão at the end (bo-TÃW̃).
não: nasal vowel (NÃW̃), not like English now.