The above may work but you might also have to right click on the outer edge of the window and chose the View option to get the others back as it is the View option that has been disabled while the tab showing running processes is running.
This problem is easy to fix, the Task Manager is running in "tiny footprint mode". To correct this, simply double-click on the border around the window and it will return to its normal state. To place task manager back in "tiny footprint mode" again, double click again on the border around the window.
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Task Manager