A little more information about what you’re doing would help in solving your problem.
It sounds like you’ve made a selection of an area that you’d like to edit, then you are clicking the "Add layer mask" button at the bottom of the Layers palette. When you do this, the part of the image you want to edit "goes away" (becomes transparent?).
If this is the situation, the solution may be as simple as simply inverting the selection or the layer mask.
One way to do this is to Alt+click on the layer mask icon in the layers palette. This should display the layer mask itself. Then you can use Image>Adjustments>Invert (or just Ctrl+I) to invert the layer mask. Now click back on the image icon in the layers palette. This should make the image visible and you should see that the part of the image that "went away" is visible and the part that was visible has now "gone away"
I’m not completely sure if this is the problem you are experiencing. If not, try and post some additonal information about what you are doing and what the problem is.
