You’re working on some changes to your website, so you put-up a message saying that the site will be down for a few hours. Your users won’t like it, but it’s necessary sometimes.
If you have to block access to your site while you work on it, make sure you include important information like, for instance, an estimate of how long the site will be down.
When you take your site down for maintenance, it’s also a good idea to give your users something to do. Give them some links, YouTube videos, etc. Make sure that, despite not being able to access your normal content, their visit was worth their time.
Here’s an example of a good downtime page:
Be sure to check out Michael Martin’s great article: 6 Ways to Help When Your Blog Is Down.
