Hello all :-), I have been using the Gradient tool in my Illustrations for 2-3 years & many times I had faced the same problem that Gradient tool bar wasnt showing up on the object & I wouldnt know how I might reactivate it. But finally I find the reason.
It called Gradient Annotator.( I didnt even know that this tool was available!)
The Gradient Annotator is an essential part of Illustrators Gradient tool (G). Thats why its such irritating when it does disappears. If you dont see your Gradient annotator when using the Gradient tool (G) then choose
View > Show Gradient Annotator (Alt+Ctrl+G).
It might be sometimes turn it off and on by accident when using the keyboard shortcuts for grouping and ungrouping objects.
Hopefully it will be helpful to you too if youre using Gradients on the object.