I was going through my inventory, cleaning up any items that I can sell on the market. I went to the Warframe one, and attempted to sell an item. It gave me an error, and told me to refresh. I refreshed, only to come back to my normal Steam inventory, but with out my Warframe inventory. Is it a bug involving the steam inventories, or did all of my items disappear?
Don't worry, your items are still there, the Steam Inventory API is just down.
The Steam inventory API goes down all the time. It happens often during high server-loads, like during Summer/Winter sales or during big tournaments, but it also happens at less frequently at completely arbitrary times. There is nothing any of us can do but wait.
If you're really worried that your account got hacked and all your items traded away, you can check your trade history and see if all your items were recently traded away.
It's extremely unlikely though, since most people have at least one untradable item. Even if all your tradable items were stolen, you'd still have something.