You know that wooden mask that you find in some ruin, where it takes you to a place, if you wear it, to put all those masks on display? Well, I accidentally dropped it when I was in the other realm, and it teleported me back to the regular realm. And I can't find the mask on the ground. Does this mean that mask is stuck in the other realm and I'm screwed?

I'm on the 360, btw.

If it's in Sovngarde, the only way to get it back is to reload the save.
Otherwise, it's gone for good. If you had a PC you could have gotten it with a console command.

Look in your inventory

Yes.
He says he dropped it not simply unequipped it.

I know, if you dropped that wooden mask in other realm, it should return to your inventory.

It wasn't in my inventory. I just ended up loading a previous save, which was no biggie.