Inventory empty ?
You probably reccently died and asked you "Wait... I owned more stuff before" and you right. When you die all stuff that you placed on your extra inventory is dropped on a box where you died. Be quiet, this box cannot be break and cannot be pick up by other players. So if you were fighting and had your weapons in main inventory, end your fight, you will be able to get back your stuff later.
Don't remember where died ? Just open your inventory. And as you can see you got a paper with the coordinates of the place where you died. Now close your inventory and press the [F3] key, you can now see your position (circled in red on the picture). Use your coordinates to find and pick up your stuff.
If you lost your stuff in an unreachable location, just explain your problem in the chat even if nobody is connected. I will see it in the next hours in messages sent list.
Oh and don't forget to put the paper in the little trash can in your inventory when you've found your items.
See also: Inventory