I was originally having that problem. I saw a couple other people posting that they had uninstalled then reinstalled the client, which allowed them to get back in. I tried that and it worked for me, but I have seen posts reporting both success and failure with that option.
For now, that seems to be your only recourse. Blizz has posted about it in the Community Forum, suggesting that reinstalling is your best current option, and that they are looking into the issue: