There's usually a malware related reason for this to happen so you need to run any scanners you have and let them fix any problems. If you don't have any, download MalwareBytes from http://www.malwarebytes.com Malware changes the Registry setting to prevent you stopping processes in Task Manager.
To get access to Task Manager back again go Start then Run and in the open box, type regedit then hit Enter. Navigate as you would in My Computer to HKEY_CURRENT_USER>Software>Microsoft>Windows>CurrentVersion>Policies>System
In the right hand pane, find and delete the DWORD DisableTaskMgr. Leave the Registry from the File menu and restart the computer.