A possible cause is when the SQL database cannot be accessed by the website. This can be caused when the SQL server service is restarted whilst the website was live.
To resolve, it is advised that you should recycle the DotNetNukeWebsite application pool within IIS.
Depending upon your configuration this is usually located in IIS under Application Pools -> DefaultAppPool-> DotNetNukeWebsite. Right click and select Recycle. After a short time the web site should then be available.