Usually, it means problems with a configuration in Startup.cs - the most common problems include an issue with DB itself, an issue with migrations (if you are using Code First approach), problems with appsettings.js, problems with Social Logins credentials (like missing SecretKey) or some other initialization issues.
To see the real error you need to enable ASP.NET Core logging in the Web.Config.
Modify aspNetCore element of your Web.Config file by setting stdLogEnabled="true" and stdoutLogFile=".\logs\stdout".
You may provide a different path but it has to be relative to your root folder and must start with ".\..."
After changing the configuration you need to create the sub folders in your root folder by using built-in File manager or FTP.
Run your website and you should see the log files appearing in the specified folder. Investigate these log files for the startup errors.