Where did all my addons go?

Alright, I noticed a few friends having this problem. I told them to restart Steam.
Then, I closed GMod to play another game, I come back and I notice ALL of my addons are missing. Having to re download them all (like their files deleted locally) or something is one thing, but I won’t actually be able to find them again to subscribe now!

There’s already a thread regarding this issue.