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.

Sorry, where do I find the CC's? I am a techno dinosaur so you might have to go full boomer dummy mode for me :p

So from your screenshot the one with the little star looking symbol is a CC pattern. If you haven't installed it as a sim3pack or a package it could have came in with a lot or a sim that you have installed.

These are some basic guides to help you dealing with bad CC that can cause issues:

This guide shows you what your Icons are https://sims3.crinrict.com/en/2012/...d-launcher.html
This talks about what I was saying about patterns made with CAP https://sims3.crinrict.com/en/2014/...-materials.html
This will help you if the CC came on a Sim https://sims3.crinrict.com/en/2011/...-came-with.html
This will help you find the CC https://sims3.crinrict.com/en/2011/...om-content.html