You may have inadvertently closed the brush by clicking on its icon on the control panel. If the brush is active, you should see the round button on your image where you first applied the brush.
If this is the case, just click on the Adjustment Brush icon again, and then on the round button on your image to select the previous brush, if there is one, or just create a new one by brushing on the image.
Also, make sure you did not accidentally click on Invert Mask - this would make all adjustments affect the areas that you did not brush.
You can see the area that will be affected by the brush settings if you press the letter O on your keyboard while the brush is active. It will turn the selected areas red.
Rafael has it pretty well imo. If the buttons disappear the "H" key will turn the buttons on and off
Thanks Rafael and Ian
After lots of trial and error, I ended up deleting the preferences folder in the Lightroom program folders and re-starting the program. All is now well with the world.
Many thanks for your suggestions.