If you have selected a section of the image, it becomes the active part being edited. Go to the top menu [Select][All] (or [None]) to get the full image back to edit mode.
When done with a toll I usually just select the next tool I need to use, or the Rectangle if I don't have a next tool to use. For example, if you add a Text Box, then it will keep trying to add new text boxes as long as the Text tool is selected. So when I'm done with Text, I click on the Rectangle tool.
BTW - when you add a Text Box, it is added as a new layer just big enough to hold the text. You will not be able to edit the main image while the text layer is active. To get back to the main image you can either (1) go to [Layers][Stack][Select next layer]. You can edit the main image but will not edit the text layer; or (2) go to [Image][Flatten image] to merge the text layer and the image layer (actually all the layers will merge). Now when you edit you can change both the image and the text because they are on the same layer.
