1) In the web.config try to set the umbracoConfigurationStatus to emty, this will force a upgrade process:
<appSettings>
<add key="umbracoConfigurationStatus" value="" />
2) In the Web.config check the umbracoDbDSN connectionstring. If you use a .sdf file check if the file exists. Else check the connection.
<connectionStrings>
<remove name="umbracoDbDSN" />
<add name="umbracoDbDSN" connectionString="Data Source=|DataDirectory|\Umbraco.sdf;Flush Interval=1;" providerName="System.Data.SqlServerCe.4.0" />
<!-- Important: If you're upgrading Umbraco, do not clear the connection string / provider name during your web.config merge. -->
</connectionStrings>
3) Your database file is corrupted. restore the .sdf file from a backup, or try the repair tool. http://sqlcecmd.codeplex.com/ see https://technet.microsoft.com/en-us/library/ms172420.aspx
Or the database file is locked or somethings by another process, restart the computer