Well, I somehow got rid of the little sidebar a long time ago, so I need help bringing that back, but if I cant, well, my cat stepped on my keyboard and did some combination to change it to the selection tool and now I cant draw.
The ‘little sidebar’ is (I suspect) the Toolbox. You can get it back with Settings -> Dockers -> click the little checkbox next to the Toolbox item in the dropdown list of dockers that appears. It should go back to where it used to be but if not, you can grab it by the thick bar at the top and drag it somewhere suitable.
The usual keyboard shortcut for the Freehand Brush Tool is ‘B’.
Cats often do things like this so I suggest that you show it this page from the manual: https://docs.krita.org/en/reference_manual/main_menu/settings_menu.html?highlight=keyboard%20shortcuts#configure-shortcuts and tell it that it has to set the shortcuts back to normal before it gets fed.