Power Shell script to stop the application pool in IIS 6.0:-

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$appPool = get-wmiobject -namespace "root\MicrosoftIISv2" -class "IIsApplicationPool" | where-object {$_.Name -eq "W3SVC/AppPools/SharePoint - 80"}
$appPool.Stop()

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Power Shell script to start the application pool in IIS 6.0 :-

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$appPool = get-wmiobject -namespace "root\MicrosoftIISv2" -class "IIsApplicationPool" | where-object {$_.Name -eq "W3SVC/AppPools/SharePoint - 80"}
$appPool.Start()

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** need to change the name of the application pool (sharepoint - 80) as per in your farm.