Hi, I don't know what happened, and I think I probably did something really stupid without knowing what I actually did (because I'm technologically challenged and a boob), but after trying to figure out how The Sims 3 Create a Pattern tool works (which I failed at that), when I went back into CAS, all the patterns have disappeared; apparently deleted.

What can I do to get them back?

That is strange. Did you add a new pattern to your game that you created with CAP?

Yes, I think I did

Go take it out. You might have caused yourself the materializing patterns issue.
So patterns made in cap will do that unless you fix them. Here is a great tutorial on how to fix them after you create them with CAP.
https://ktarsims.tumblr.com/post/15...oarder-tutorial

Here is a thread on it. https://modthesims.info/t/574820

Where do Iook to take it out; where would the file be?

well if you didn't convert it to a .package and place it in as a mod. Check your downloads folder.
If you have the launcher look in the downloads for a pattern.

Nope, still didn't come back :/

I've just attached a screenshot of it. It only has 2 categories; "Misc." and "Favorites".

Remove all CC patterns. Take them out of any of your game folders.
Delete your caches.

Also, let me ask this did you install any CC that would have come with patterns. Like on a Sim or in a lot?
You might have to remove all CC. I would start with your latest CC and work your way back until your patterns come back. Make sure to clean your caches at each instanse of doing this.

When I have something like this happen I use a small empty CC world that I use just for testing. This way I can get in quick and test things. I put a lot down with furntiure and one sim in the whole world it's my tester world. So quick loading and saving to test.