I think there were some hiccups yesterday. I'm not saying this was your same problem, however here is an issue I had and the solution.
While in the design tool I noticed my elements I was designing with stopped showing in the little preview product view on the bottom right. I had to back out of the tool, go into my Saved Designs page and reload the product to continue to work on the design.
Sometimes there are hiccups with the design tool. You just have to be patient and maybe take a small break (or longer if there is truly a big issue at hand). After that one product design in the design tool was completed. I had no more issue.
