Without seeing your site, it could be a temporary template. Go to wp-admin and click on Appearance -> Themes. The top of that screen will show the Current Theme. Below that are the themes available to you. Experiment with the themes to see if one of them is what your site usually looks like.
Or it could be that there’s a Maintenance Mode plugin installed. Look at wp-admin for Plugins to see if one’s installed. Or check the Settings section for a Maintenance Mode setting which shows up when there’s a maintenance mode plugin.