Hi Natalie,
I had similar problems several times .... (like, I think, nearly everybody using Photoshop

)
In my case, deleting the Photoshop Preferences File always solved the problem ... You can find a very clear article about the 'Why' and 'How to'
here
Photoshop creates the 'several Folders' in the Preset Folder when installed... but loads the 'Default' only at startup, enabling you to start working immediately, but decreasing, this way, the loading time.... The rest, is there to be loaded, appended, used on necessity...
Usually, once you load something in addition to the Default Presets, Photoshop asks you the to 'Append' the new set to the one you are currently using (otherwise you won't be able to use any of its content).... If you click OK, it will be added to it ..... the new combination will be saved in the Preferences File when you close
PS ....
and .... loaded (
as saved) next time you start
PS.
If your Presets get too 'crowded' and you'd like to go back to the Default Settings, in the current Preset Menu, click on
Reset (Brushes...Actions....Tools...whatever) and you'll be brought back to the initial Photoshop Default Preset ....
Now (talking from personal experience here...

).... having deleted all the Brushes, your Brushes Preset is 'Empty' .... sooo .... unless you saved the original Presets folder somewhere else before experimenting ..... so you can copy it back .... you'll have to start from scratch .... meaning re-installing Photoshop!
Hope this could help...