First of all you need to check Plesk registry sub tree
find necessary component and compare it's records with default ones.
Default records are in attached file packages.reg.
If there everything is correct you need to check whether all the .dll files are present in %plesk_bin% that is used by Plesk to control some component. You can find .dll name at key
HKEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\Config\Packages\mailserver\mailenable\dll == 'C:\Program Files\SWsoft\Plesk\admin\bin\mailenableproviderw.dll'
If .dll is not present in %plesk_bin% you need to recover it.
At the end you need to log on to the Plesk Control Panel > Server > Components Management > click on 'Refresh' button. Component should appear.