I'm just wondering if my characters are saved on the CoTH database - even though I only played for a day, I don't want to lose all the items I got. A simple yes or no will do. Thanks!

They are, don't worry.

At best you'll lose your cache for stuff like Add-ons, but characters and their possessions should be safe.
The characters and all their items or spells etc. has nothing to do with your files. It is all completely stored on the server.
So yes, they will be safe.