Adobe Photoshop is an insanely large, incredibly customizable application. Sometimes some of the (many) tools may stop working as you expect, either due to something you did intentionally or accidentally, or due to a bug. Either way, resetting a tool to its default settings can often solve problems.
Reset an individual tool
To revert an individual tool to its default options, select it from the Tools toolbar. Then right click on the tool icon in the tool options bar and select “Reset Tool”
How to reset all your tools at once
To reset all tools at once, right click on the icon of the tool currently selected in the Tool Options, select ‘Reset All Tools’ and then click ‘OK’.
Using the Reset All Tools option also resets all tool selections in the Toolbar to their default values.
How to reset your paint samples
To reset the paint swatches to their default values, click the little black and white swatch icon below them. You can also press the hotkey “D”.
How to Reset Photoshop to Defaults
If things continue to behave abnormally with all your tools, you can use the nuclear option and reset Photoshop to its default settings.
To do this, go to Edit > Preferences > General (on a Mac, this is Photoshop CC > Preferences > General). Click the “Reset preferences on exit” button.
Click “OK” and restart Photoshop. Everything is now back to its original settings.
There should be no reason to reinstall Photoshop on your Windows PC or Mac as this button completely resets the settings. However, if you’re still having issues, you can try a full reinstall.