trouble using brush tool with PNG files in Photoshop

[mattyvt2066]

Hi guys, new to photoshop, im trying to use the black brush tool within a mask to erase part of an image, however when i activate the mask and activate the brush tool with 100percent opacity and hardness, nothing happens. When i try to use the brush tool on the orginal image layer, again nothing also happens. Is there some setting i need to change? do i need to change the file of the image?

ive atached a screen shot of what it looks like when i try using the brush tool on the image

[c.pfaffenbichler]

Could you please post a screenshot with the pertinent Panels (Toolbox, Layers, Channels, Options Bar, …) visible? 

[mattyvt2066]

[c.pfaffenbichler]

There is no Layer Mask there. 

Also the Options Bar and Toolbox are missing in the screenshot so no idea what the Brush Tool’s settings are … 

[mattyvt2066]

[c.pfaffenbichler]

The Tool is set to Blend Mode »Lighten«. 

Edit: 

[mattyvt2066]

oh wow, i just changed it back to normal and it worked! thankyou so much !!!