If task manager is not open then it can be recover by 2 ways

1st way :
In cmd prompt type : REG add HKCU\Software\Microsoft\Windows\CurrentVersion\Policies\System /v DisableTaskMgr /t REG@DWORD /d 0 /f  and press enter nd exit.

2nd way :
Type gpedit.msc on run on that administrative templates->system->ctl+alt+delete->remove taskmanager = not configured then exit

Then now you can use the taskmanager Ensoy!!!!