Yesterday, I was running Gmod for approximately five hours. No problems.
Today, I tried running Gmod, and the first thing I noticed was that none of the workshop maps showed up in the map screen.
“Oh well” I said, and ran with gm_construct.
When I got into the game I was amazed how quickly the spawn menu loaded. Just to find out that it didn’t load any of the addons.
So I restarted Steam, restarted Gmod and checked the addons menu.
All the addons properly show up in the menu.
Went back ingame. Nothing had changed, the addons didn’t load.