How To Set SharePoint Site To Read only Mode

To Set Read Only Mode:

# Get the Web Application
$WebApp=Get-SPWebApplication "http://mywebsite"

#Loop through all site collections and Set read only status
    foreach ($SPsite in $webApp.Sites)
    {
       $SPsite.ReadOnly = $true
       #Or you can use: Set-SPSiteAdministration -LockState "Lock" -Identity $SPsite.url
    }


To Revert Back

# Get the Web Application
$WebApp=Get-SPWebApplication "http://mywebsite"

#Loop through all site collections and Set read only status
    foreach ($SPsite in $webApp.Sites)
    {
       $SPsite.ReadOnly = $false
       #Or you can use: Set-SPSiteAdministration -LockState "Unlock" -Identity $SPsite.url
    }