Web Part Error

One of the SharePoint out-of-the box (created with SharePoint tools and no custom code) web parts has suddenly stopped working, offering only this information to debug:

Web Part Error: A Web Part or Web Form Control on this Page cannot be displayed or   imported. The type could not be found or it is not registered as safe.

I am lost at to why all the other parts (Custom code and just content) seem to load, but just this one is looking for type/dll throwing the exception.

Where to start debugging and what pieces of SharePoint I have to learn now?

I checked the log on where the app is hosted and found no issues directly pointing to the issue.

Not many issues at all except: Fialed to compile audiences. Can this be an issue for a text.

While adding custom web part to a sharepoint site In the Web.config file, add the following element under the SafeControls element:

<SafeControl Assembly="AssemblyName, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" Namespace="NamespaceName" TypeName="*" Safe="True" />

For more refer this link http://support.microsoft.com/kb/939306