To Invert, You Select the Layer, and press CTRL+I
You might only be Viewing certain Colors, Press CTRL+~ to view RGB
(CTRL+1, CTRL+2, and CTRL+3 will enable you to view in grayscale, only spefic colors)
You might be in Free Transform Mode, If you've press CTRL+T then you need to click on the cursor at the top left of the sidebar, then hit apply
- cant think of much else... Good Luck