Enabling Hidden Task Tracks
All disabled task tracks appear in the Tracks menu > Disabled Tracks list.
To re-enable a disabled task track and append it to the Task Timeline panel, click on one of the items in the list.
To re-enable all disabled task tracks, click the Enable All Tracks button 
from the toolstrip. All items in the list will be removed and the tracks will append to the bottom of the Task Timeline panel.
See Also
Disabling Undesirable Task Tracks