$farm  = Get-SPFarm
$disabledTimers = $farm.TimerService.Instances | where {$_.Status -ne "Online"}
if ($disabledTimers -ne $null)
{
    foreach ($timer in $disabledTimers)
    {
        Write-Host "Timer service instance on server " $timer.Server.Name " is not Online. Current status:" $timer.Status
        Write-Host "Attempting to set the status of the service instance to online"
        $timer.Status = [Microsoft.SharePoint.Administration.SPObjectStatus]::Online
        $timer.Update()
    }
}
else
{
    Write-Host "All Timer Service Instances in the farm are online! No problems found"
}