$offlinedisks = Get-Disk | Where-Object OperationalStatus -EQ ‘Offline’
foreach ($disk in $offlinedisks)
{
Set-Disk -Number $disk.Number -IsOffline $false
Set-Disk -Number $disk.Number -IsReadOnly $false
}

Thanks Barrie, that's a great suggestion.

$offlinedisks = get-disk | where OperationalStatus -EQ offline

foreach ($disk in $offlinedisks)

{Set-Disk -Number $disk.Number -IsOffline $false

Set-Disk -Number $disk.Number -IsReadOnly $false}