Layer Mask Erasing itself?

Hi.. I had the same issue this morning, and while looking for a solution, I found this post to which I would like add a detail that can be useful for these having the same issue.
When following @davescm advices, the gradient was still not working and erased my previous grandient.
I then changed the color in my palette (General color, not the gradient one) from white to black and Tada!! Magic it worked again.

The gradient tool blends two "solid" colors so always has overidden the previous gradient - unless you :

a. Change the blending mode in the options bar

b. Change the opacity in the options bar

c. As described by Silkrooster, choose a gradient with transparency.

Dave

Make sure you use the transparent to foreground gradient and not the black to white gradient. Then you can apply multiple gradients to the same mask. You can then use the foreground color to change how the gradient will behave.

Another option is to change the blend mode of the brush.

Can you post a screenshot of your workspace with the Layers panel opened so we can see what you're working with?