I was playing Sajan, the monk, and was fighting an ogre.  I was going to use my +1 frost sling, and I revealed my belt of dexterity.  I then clicked the red 'x' on the side to undo because I was going to see if my monk ability was any better, but when I tapped the button, the belt disappeared.  When I finished the level, the belt was still gone as though I had banished it; i don't think that the belt even has a banish ability...

Any help available with this?

Your best bet is to send support@obsidian.net an email if you require further assistance.

They'll need to know your PFID (at the bottom of  Options --> Settings) what happened, and what you would like them to do.  If you can specify the name of the character the lost the card, they may be able to reinsert it into inventory, otherwise it may just be possible to drop a new copy into your party stash.