Let's begin by clicking on the "Start" menu on the taskbar. Followed which, click on "Run". In the "Run" window, type: regedit and click on "OK" Navigate to: My Computer \ HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System In the right-pane, right click and delete the value, "DisableTaskMgr" [Deleting this value from the Registry Editor would make sure that the task bar remains enabled at all times]. Now Navigate to: My Computer \ HKEY_LOCAL_MACHINE \Software \ Microsoft \ Windows \ CurrentVersion \ Policies\ System Again, here in the right-pane, right click and delete the value, "DisableTaskMgr". After this, close the Registry Editor and reboot the system. Now the next time, you want to access the Task Manager you can do it without any difficulty.
Access to your Task Manager usually gets disabled if your PC is affected by some Trojans or Malwares, with the result you are stuck up and can't end processes. When the situation gets messy, all you need to do is to make these changes to your Registry Editor to enable you to access the Task Manager again.