How do I stop the player moving and looking around when UI appears?

I basically want to stop the player doing anything when the UI appears of the bottom of this script:

public static int NumNotesRemaining = 0;
public GameObject NotesFoundPanel;

// Start is called before the first frame update
void Start()
{
    NumNotesRemaining += 1;
}

void OnTriggerEnter(Collider other)
{
    if (!other.CompareTag("Player")) return;

    NumNotesRemaining -= 1;
    gameObject.SetActive(false);

    if(NumNotesRemaining <= 0)
    {
        NotesFoundPanel.SetActive(true);
    }
}

Where I make the UI appear I need to disable player movement so that the player can click to remove the UI, then I need to player movement to be enabled again.

You could set the timescale to 0 while the UI is showing and set it back to 1 when the UI disappears.