Unity C# Invoke Function Freezes Program

You seem to be under the impression that this line of code asks to do multiple things:

Invoke("bulletplus", 0.5f);

• Schedule the method named "bulletplus" to be called 0.5 seconds from now.
• Pause the execution of the current method we called it from, and let the rest of the engine continue running its updates and rendering for the frame.
• Don't resume this method until "bulletplus" has run to completion. After that, resume the calling method with its prior state on the next line after the Invoke.

In actual fact, it only does the first line, in bold. Pausing and resuming the calling method are not features it offers according to the documentation. You can verify this yourself with a simple test:

void Start()
{
    Debug.LogFormat("{0}: Start starts at {1}", ++callOrder, Time.time);
    Invoke("InvokeReporter", 0.5f);
    Debug.LogFormat("{0}: Start finishes at {1}", ++callOrder, Time.time);
}

void InvokeReporter() {
    Debug.LogFormat("{0}: Invoked method runs at {1}", ++callOrder, Time.time);
}

You can see Invoke does not add any delay between the first and second halves of Start - they run back-to-back in the same frame.

So, as was pointed out to you in the comments, this is an infinite loop:

while(ammo <= 9)
{
     Invoke("bulletplus", 0.5f);
}

Scheduling an invocation of "bulletplus" to happen later does not increase the value of the ammo variable now. So if ammo was less than 10 when we entered the loop, it will still be less than 10 no matter how many times we repeat the loop, and we can never escape.

Apart from a coroutine hitting a yield statement, Unity executes each MonoBehaviour message method completely, from top to bottom or the first return statement, before moving on to the next object/message or proceeding with any of the other work of the frame.

So if your Update method gets stuck in this infinite loop and never returns, then Unity is stuck executing it endlessly, unable to move onto the next script that needs updating, or draw a new frame, or process new input. Your game hangs because the engine can't get to updating the window.

It looks like what you want is a coroutine something like this:

Vector2 rotation = new Vector2 (0, 0);
public float rotationSpeed = 3;
public Rigidbody projectilePrefab;

public Text ammoText;
public int ammo = 10;
bool reloadInProgress = false;

void Update () {
    // Rotation
    rotation.x += -Input.GetAxis ("Mouse Y") * rotationSpeed;
    rotation.x = Mathf.Clamp(rotation.x, -70, 70);
    rotation.y += Input.GetAxis("Mouse X") * rototationSpeed;
    rotation.y %= 360.0f;        
    transform.eulerAngles = rotation;

   
    // Can't shoot or start another reload while already reloading.
    if(reloadInProgress)
        return;
    

    if(Input.GetMouseButtonDown(0) && ammo >= 1)
    {
        UpdateAmmo(-1);
        Rigidbody projectile = Instantiate(
                                 projectilePrefab,
                                 transform.position,
                                 transform.rotation);
        projectile.velocity = transform.forward * 50f;
    }

    // Reload
    if(Input.GetKeyDown(KeyCode.G) && ammo <= 9)
    {
        // Start a method that can take turns with the engine,
        // pausing for multiple frames then resuming when needed.
        StartCoroutine(Reload());
    }
}

IEnumerator Reload() {
    reloadInProgress = true;

    while(ammo <= 9) {
        // Put this coroutine on hold for a period of time, 
        // while letting the rest of the game run.
        yield return new WaitForSeconds(0.5f);
        // Then resume our execution on the next line:
        UpdateAmmo(+1);
    }

    reloadInProgress = false;
}

// Let's do this update work only when the ammo changes, not every frame.
void UpdateAmmo(int ammoChange) {
    ammo += ammoChange;
    ammoText.text = "Ammo: " + ammo.ToString();

    if(ammo >= 7) {
        ammoText.color = Color.green;
    } else if(ammo >= 4) {
        ammoText.color = Color.yellow;
    } else {
        ammoText.color = Color.red;
    }
}