Fans trying to make reservations for the 2016 Worldcon today reported it was not possible to book a room for MidAmeriCon in any of their hotels.
The committee posted this explanation on the MAC II webpage.
Due to unprecedented demand for hotel rooms at a Worldcon, our original hotel blocks were filled more quickly than expected. We are in the process of expanding our current blocks to accommodate all of you! We will announce when additional space becomes available, and you might want to check back in a few days as well.
If you have a specific issue with your reservation, write to email@example.com, but please note that they are already aware of the filled room blocks and are working tirelessly to alleviate this situation.