Please try these steps:
1. Try Plesk Reconfigurator (Start ->All Programs->SWsoft->Plesk Reconfigurator->Repair Plesk Installation)
2. Start->Control Panel->Add/Remove Programs->Plesk 7.5.5 for Windows->Repair
3. Regedit->Go to the HKEY_LOCAL_MACHINE\SOFTWARE\PLESK\PSA Config\PSA Key-> ensure that user MachineName\psaadm has at least read permissions to each subtree
4. Check Computer Management, does service Plesk Control Panel and Plesk Management services started (also you can check it by black X in tray). Try restart them. If there will be any error as temporary measure you can assign authority to start service to Local System (Not recommended)
5. Check any errors in event logs
You can also provide additional information (error description)
John S.G.