Из-за жара началось головокружение.

Из-за means because of, due to, or as a result of. In this sentence, it introduces the cause of the dizziness.

A very important grammar point: из-за is a preposition that normally takes the genitive case.

So:

• из-за жара = because of the fever / because of the heat

Because из-за requires the genitive case.

The dictionary form is жар.
Its genitive singular form is жара.

So:

• жар = fever, heat
• из-за жара = because of the fever / heat

This is one of the most common things to notice in Russian: the preposition changes the form of the noun.

It can mean either one, depending on context.

Common meanings of жар include:

• fever
• heat
• strong bodily heat or high temperature

In a medical context, из-за жара is often understood as because of the fever.
In another context, it could mean because of the heat.

So the word itself is somewhat flexible, and context tells you which meaning is intended.

Началось is the past tense of начаться, which means to begin, to start, or to come on.

Here it means that the dizziness started or began.

So:

• начаться = to begin, to start
• началось = began / started

This is a very natural choice when talking about the onset of a symptom.

The -сь is the short form of -ся, the reflexive postfix.

It tells you that this is the verb начаться, not начать.

Compare:

• начать = to begin something
  
  • Он начал разговор. = He began the conversation.
• начаться = to begin / to start
  
  • Разговор начался. = The conversation began.

So in your sentence:

• головокружение началось = the dizziness began

Without -ся/-сь, the verb would usually need a direct object.

Because the verb agrees with головокружение, which is a neuter singular noun.

In the past tense, Russian verbs agree in gender and number:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

Since головокружение is neuter, the correct form is:

• головокружение началось

Yes. Головокружение is the grammatical subject.

Literally, the structure is close to:

• Because of the fever, dizziness began.

Even though English often prefers something like The fever caused dizziness or Dizziness started because of the fever, Russian is very comfortable making головокружение the subject and using начаться.

Головокружение means dizziness or vertigo.

It is built from parts related to:

• голова = head
• кружить / кружение = to spin / spinning

So the word gives the image of head-spinning, which matches the meaning of dizziness.

It is a neuter noun, which is why the verb is началось.

Because началось emphasizes the start or onset of the symptom.

Compare:

• началось головокружение = dizziness began / came on
• появилось головокружение = dizziness appeared
• было головокружение = there was dizziness / dizziness was present

So началось specifically focuses on the moment it started, which is very natural in medical or descriptive contexts.

Yes, very often.

Из-за frequently introduces a cause that is unwanted, problematic, or negative:

• из-за дождя = because of the rain
• из-за болезни = because of the illness
• из-за ошибки = because of the mistake

So it fits very naturally with something unpleasant like fever and dizziness.

That said, it is not limited only to negative situations, but negative causes are especially common.

Yes. Russian word order is flexible.

Your sentence:

• Из-за жара началось головокружение.

A different order could be:

• Головокружение началось из-за жара.

Both are grammatical. The difference is mainly about emphasis.

• Из-за жара... puts the cause first.
• Головокружение началось... puts the symptom/event first.

Starting with из-за жара can sound a little more like you are setting up the reason first.

Yes, in many contexts you could.

• из-за жара = because of the fever / due to the fever
• от жара = from the fever / because of the fever

Both can be natural, but the nuance is a little different.

Very roughly:

• из-за often points to a cause in a broader because of / due to sense
• от often sounds more like a direct physical result: from

So головокружение от жара can sound very natural too, especially in medical or bodily descriptions.

Russian often leaves that unstated if the context already makes it clear.

This sentence simply states that dizziness began because of the fever. It does not explicitly say whose dizziness it is.

If needed, Russian could make it more personal:

• У меня из-за жара началось головокружение. = I started feeling dizzy because of the fever.
  Literally: At me, because of the fever, dizziness began.

But without у меня, the sentence is still perfectly natural if the person is already understood from context.