While rare, there are occasions while updating a plugin or theme things may not go smoothly. I was trying to update a theme on one of my (many) WordPress sites. The update was successful, however the server hiccuped before maintenance mode was turned off. So the site was stuck in maintenance mode, which means you can not access the front end or even your dashboard. I do believe though there is a some type of safety timeout threshold in place so that your not permanently locked out of your dashboard. I had a similar issue while trying to do a WordPress core update and was able to get back in a after a few minutes. Nonetheless, you can get out Maintenance Mode immediately by connecting to the server through FTP or SSH and removing the file .maintenance from the root folder of your WordPress install. Once that file is gone you should be able to access your dashboard try the update again.