Repeatedly checks whether a certain IP is reachable and shows a popup if it's not.

offline-check.ps1

$ip = "192.168.1.1"
Add-Type -AssemblyName System.Windows.Forms

while ($true) {
    if (!(Test-Connection $ip -Count 1 -Quiet)) {
        [System.Windows.Forms.MessageBox]::Show(
            "Device $ip is offline!",
            "Network Alert",
            [System.Windows.Forms.MessageBoxButtons]::OK,
            [System.Windows.Forms.MessageBoxIcon]::Warning
        ) | Out-Null
    }
    Start-Sleep -Seconds 5
}