Find Out Why a Windows Machine Rebooted with PowerShell

<#
.SYNOPSIS
    Shows when a computer shut down or restarted, who or what asked for it, and whether it was clean.
.DESCRIPTION
    Reads the System event log with Get-WinEvent and returns one object per event:
      1074  User32                          A process or user asked for a shutdown or restart (with reason)
      6006  EventLog                        The event log service stopped: a clean shutdown
      6005  EventLog                        The event log service started: the machine booted
      6008  EventLog                        The previous shutdown was unexpected
      41    Microsoft-Windows-Kernel-Power  The system rebooted without shutting down cleanly
    Works against remote computers over the Remote Event Log Management firewall rules.
.PARAMETER ComputerName
    One or more computers. Defaults to this one.
.PARAMETER Days
    How far back to look. Default: 7.
.PARAMETER MaxEvents
    Cap on events per computer. Default: 500.
.PARAMETER Credential
    Credentials for remote computers.
.EXAMPLE
    .\Get-RebootHistory.ps1 -Days 30
.EXAMPLE
    .\Get-RebootHistory.ps1 -ComputerName PC-0142, SRV-APP01 | Where-Object Type -eq 'Unexpected'
#>
[CmdletBinding()]
param(
    [Parameter(ValueFromPipeline, ValueFromPipelineByPropertyName)]
    [Alias('CN', 'DNSHostName')]
    [string[]]$ComputerName = $env:COMPUTERNAME,
    [ValidateRange(1, 3650)][int]$Days = 7,
    [ValidateRange(1, 100000)][int]$MaxEvents = 500,
    [pscredential]$Credential
)

begin {
    $filter = @{
        LogName      = 'System'
        ProviderName = 'User32', 'EventLog', 'Microsoft-Windows-Kernel-Power'
        Id           = 1074, 6005, 6006, 6008, 41
        StartTime    = (Get-Date).AddDays(-$Days)
    }
    # The hashtable matches any provider with any ID, so double-check the pairs we actually want.
    $wanted = @{ 1074 = 'User32'; 6005 = 'EventLog'; 6006 = 'EventLog'; 6008 = 'EventLog'; 41 = 'Microsoft-Windows-Kernel-Power' }
}

process {
    foreach ($computer in $ComputerName) {
        $params = @{ FilterHashtable = $filter; MaxEvents = $MaxEvents; ErrorAction = 'Stop' }
        if ($computer -notin @('.', 'localhost', $env:COMPUTERNAME)) { $params.ComputerName = $computer }
        if ($Credential) { $params.Credential = $Credential }

        try {
            $events = Get-WinEvent @params
        }
        catch {
            if ($_.FullyQualifiedErrorId -like 'NoMatchingEventsFound*') {
                Write-Verbose "$computer : no shutdown or startup events in the last $Days day(s)"
            }
            else {
                Write-Warning "$computer : $($_.Exception.Message)"
            }
            continue
        }

        foreach ($e in $events) {
            if ($wanted[$e.Id] -ne $e.ProviderName) { continue }

            $type = $null; $user = $null; $process = $null; $reason = $null
            switch ($e.Id) {
                1074 {
                    # Properties: 0 process, 1 computer, 2 reason, 3 reason code, 4 shutdown type, 5 comment, 6 user
                    $p       = $e.Properties.Value
                    $type    = (Get-Culture).TextInfo.ToTitleCase([string]$p[4])
                    $process = ($p[0] -replace '\s*\(.*\)$', '')
                    $reason  = (@($p[2], $p[5]) | Where-Object { $_ }) -join ' | '
                    $user    = $p[6]
                }
                6005 { $type = 'Startup' }
                6006 { $type = 'Clean shutdown' }
                6008 { $type = 'Unexpected'; $reason = 'Previous shutdown was unexpected' }
                41   { $type = 'Unexpected'; $reason = 'Kernel-Power 41: rebooted without a clean shutdown' }
            }

            [pscustomobject]@{
                ComputerName = $e.MachineName
                TimeCreated  = $e.TimeCreated
                EventId      = $e.Id
                Type         = $type
                User         = $user
                Process      = $process
                Reason       = $reason
            }
        }
    }
}