Whenever an error has occured in SharePoint web site, it basically displays some generic error messages like "An unknown error has occurred" or "An unhandled exception has occurred". To troubleshoot and resolve these errors, more descriptive error messages will be needed which notify the exact location of the error.
To get the real error messages or more descriptive error messages, following changes needs to be done in the web.config file of the SharePoint web site:
1) Set CallStack="true" and MaxControls="200" in the "SafeMode" tag that can be found under "SharePoint" tag.
<SharePoint><SafeMode MaxControls="200" CallStack="true" .................
2) Set mode="Off" in the "customErrors" tag.
<customErrors mode="Off" >
After doing these changes, just refresh the web page which was getting an error and it will display the more descriptive error message along with some stacktrace information.
If you still get the same generic error message then just reset the IIS and refresh the web page again, it will definetly display the more descriptive error message.