Se l’applicazione si blocca, devo riavviare il computer.

Italian often drops the final vowel of an article before a word that begins with a vowel.
So la applicazione becomes l’applicazione.

This is called elision.

You will see the same thing in examples like:

• l’amica
• l’idea
• l’ora

Because applicazione is feminine singular, the full article is la, and before a vowel it becomes l’.

Se means if in Italian.

In this sentence, Se l’applicazione si blocca introduces the condition: if the application freezes/crashes.

It works much like English if:

• Se piove, resto a casa. = If it rains, I stay home.
• Se ho tempo, ti chiamo. = If I have time, I’ll call you.

The verb here is bloccarsi, which means to freeze, to get stuck, or to stop responding.

The si is part of this reflexive/pronominal form. In technology contexts, Italian often uses bloccarsi to say that a device, app, or system freezes.

So:

• l’applicazione si blocca = the application freezes / gets stuck

Compare:

• bloccare = to block something
• bloccarsi = to become blocked / to freeze

For example:

• Il sistema si blocca spesso. = The system freezes often.
• Ho bloccato il numero. = I blocked the number.

It is in the present tense because Italian commonly uses the present in if clauses when talking about general situations or real possibilities.

So Se l’applicazione si blocca literally uses the present, just like:

• If the app freezes
• When the app freezes

Italian often uses the present where English may also use the present:

• Se piove, resto a casa.
• Se arriva tardi, iniziamo senza di lui.

This sentence expresses a general rule or usual response: if that happens, this is what I have to do.

Devo is the first person singular of dovere: I must / I have to.

The subject is understood rather than stated explicitly:

• (Io) devo riavviare il computer.

Italian often leaves out subject pronouns when the verb ending already makes the subject clear.

Here are some forms of dovere in the present:

• io devo = I must
• tu devi = you must
• lui/lei deve = he/she must
• noi dobbiamo = we must
• voi dovete = you all must
• loro devono = they must

Dovrei would mean I should / I would have to, which is softer and more conditional.

Because after dovere, Italian normally uses an infinitive.

So:

• devo riavviare = I must restart

This is the same pattern as in English:

• I must go
• I must study
• I must restart

Other examples:

• Devo lavorare. = I have to work.
• Devo chiamarlo. = I have to call him.
• Devo uscire. = I have to go out.

Avviare means to start or to launch.
Riavviare means to restart or to reboot.

The prefix ri- often gives the idea of again.

So:

• avviare il computer = to start the computer
• riavviare il computer = to restart the computer

You see this prefix in many verbs:

• fare → rifare = do → do again
• leggere → rileggere = read → reread
• scrivere → riscrivere = write → rewrite

Italian usually uses definite articles more often than English.

So where English might say restart the computer or sometimes just talk about computer in a general practical context, Italian naturally says il computer.

Also, computer is treated as a masculine noun in Italian, so the article is il:

• il computer
• un computer

This is very common with borrowed words.

Yes, computer is completely normal Italian usage.

You may also hear:

• PC
• portatile = laptop
• desktop in some contexts

But computer is standard and natural.

So riavviare il computer is exactly what you would expect in everyday Italian.

The comma separates the if-clause from the main clause:

• Se l’applicazione si blocca,
• devo riavviare il computer.

This is very common when the conditional clause comes first.

Italian punctuation here works much like English:

• If the app freezes, I have to restart the computer.

If the order were reversed, the comma would often be omitted:

• Devo riavviare il computer se l’applicazione si blocca.

Yes. You can also say:

Devo riavviare il computer se l’applicazione si blocca.

Both versions are correct. The difference is mostly one of emphasis and flow.

• Se l’applicazione si blocca, devo riavviare il computer.
  This foregrounds the condition first.

• Devo riavviare il computer se l’applicazione si blocca.
  This starts with the action I need to take.

Yes, but the meaning changes.

• si blocca = freezes / if it freezes
  This is general or habitual, or refers to what happens when that situation occurs.

• si è bloccata = has frozen / got stuck
  This refers to a specific completed event in the past or just now.

So compare:

• Se l’applicazione si blocca, devo riavviare il computer.
  = If the application freezes, I have to restart the computer.

• L’applicazione si è bloccata, quindi devo riavviare il computer.
  = The application froze/has frozen, so I have to restart the computer.

Yes. Italian often omits subject pronouns because the verb ending already shows the subject.

So:

• devo already tells you the subject is io
• deve would suggest lui/lei
• dobbiamo would suggest noi

You could say io devo, but it is usually unnecessary unless you want emphasis:

• Io devo riavviare il computer, non tu.

In the original sentence, leaving out io sounds completely natural.