If the other item doesn't turn up in the next 24 hours you must open a case.
If the two items were one order, and the seller has used the same tracking number for both, you will have to open a return case and use the reason 'items missing'. You may need to return the item you did receive.
Don't open an 'item not received' case if the tracking shows delivery.
Have a read up about the Ebay Money Back Guarantee before you use it.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)