Clearing your Cache

Select Edit > Preferences > Network
from the menu bar across the top of your screen.  On the Network Tab, press the “Clear Cache” button.  You will then need to relog.

Once you get back, open your inventory window and type 3-4 random characters into the search box. This will speed up the reload of your inventory. Once it’s completely reloaded (the item count stops incrementing), you will hopefully find your missing item.  If not, It’s likely your item has been eated by the hungry Linden Database Monster.