There are some problems with selection tools that everyone is tripped up by sooner or later. Here is a common list:
This is such a common problem! It is due to the Marching Ants having been toggled off in some prior operation. Toggle the Marching Ants back on by typing C-t in the image window or by clicking on the Toggle Selection checkbox in the Image:View menu.
Your image contains multiple layers, and the active layer is not visible. Operations in the GIMP are applied to the active layer (for more information, see Chapter 2). Thus, the selection in your image may be visible, but the active layer may not be. Open the Layers dialog and verify that the layer you are working on is active and is visible in the image window.
Open the Paths dialog and verify that there is an active path in the Paths Palette. If there isn't, create a new one by clicking on the New Path button. Also, verify that the New Point mode button is toggled on.