Data loaded in memory is only available while the application is running. Once the application is closed, all of the data loaded will be unloaded and destroyed.
If you need to the data to be available after the application has closed you need to store it somewhere so that it can be re-loaded into memory the next time the application is started.
You could use a database to store the data. There are free ones available that you can use (like mySQL).
Or, if your application is relatively simple, you could write data into a file so that it can simply read the file the next time the application is opened to recreate the data you need.
If you are going to save data into a file, I recommend storing it in XML format because it works well with Object Oriented Programming.
Here is a link to the MSDN Documentation for the
XmlSerilizer Class that is used to serialize objects into XML. Hers is another MSDN article on the topic of
XML Serialization in the .NET Framework.
-Frinny