While you are updating your plugins, themes or WordPress core itself, your site will be in maintenance mode. WordPress does this by default and it is a safe and recommended way to update your website. After the updates are done, WordPress will automatically remove the maintenance mode.
However, in some cases, even though the updates have been performed successfully, your website will still be stuck in maintenance mode. By going to your homepage, you’ll see something like this:
No need to worry, just log into the FTP of your site (by using an FTP client like Filezilla), and delete the .maintenance file. Your site will be up again in no time!