A Google Webmaster Help thread has dozens of webmasters complaining about not being able to verify their web sites.
The first report came in yesterday afternoon. The webmaster said, "I just lost verification of all of my sites."
Google's Gary Illyes responded an hour later that the team is on it. Gary wrote:
Thanks for the reports, guys! I escalated them to the Webmaster Tools team and they are looking into it. Thanks again for reporting it!
Hopefully the issue will be fixed by the time this post goes live.
Forum discussion at Google Webmaster Help.