I tried to install some addons, after I placed those new addons into the AddOns folder (interface/addon ), something happened when I restarted the game — the addons that I also recently had disappeared.

Half of my addons were gone. So addons like Details, Skada, OneBag, Gatherer and others stopped working. The only ones that still work are GearScore,AtlasLoot and DBM. Everything else got disabled, even though it was working before.