Whenever I am in a dungeon or going somewhere, and have a companion, sometimes they will disappear for a good long while. When they do come back, all the items I have given them are gone. How do I get the items back without reloading? I don't always know when my companion is gone.
[PC answer]: Outside of using console commands or simply buying or resmithing/enchanting your gear, I don't know of a way to get the gear back but I do know of a mod that can help you avoid this situation in the future: Skyrim Nexus - Summon Followers.
This mod is extremely handy and I've made a habit of using it whenever I'm using a follower and they've either fallen too far behind or are just not in the same cell as my character. It comes from a spell tome found in front of Uthgerd in Whiterun's inn. Be aware that if you go this route, SKSE - Skyrim Script Extender is required for Summon Followers to work.
When playing the PS3 version, the followers cannot activate lifts in Dwarven dungeons and so often get stuck.
If it's just a generic dungeon, they can move in and out as is if were any other building.
The quickest way to get them back is to fast travel and they should appear right next you. If they don't, they're in a Dwarf dungeon and you need to go back to it to find them.
I have no idea why they would also lose their items.