I have had this happen, and the room IS held for you. Eventually the system sees that you're not booking the room and releases the hold (takes 3-10 minutes). Up until that point though it will look sold out if you had the last available room. I've had my nerves on edge a number of times due to exactly what you experienced and waiting for my room to be re-released so I can book it. Was particularly painful at 2am one night as I was moving my rooms around, consolidating them, before I knew member services could do this for me.
By the way, calling will do nothing to help, the cast members cannot see any in-process transactions to retrieve yours.
The ongoing/unaddressed tech glitches kill me.
Sometimes those errors seem to come from the system having trouble with the browser on our end too. I've learned not to use Firefox for this reason.