When you stop and start a project, you'll lose the view state data on your gridview.
You need to load up the data again, which it loks like you're doing when you click Button2_Click.
Add the following to perform the databind.
gridview.DataSource = Ds.Tables("MataKuliah")
gridview.DataBind();
http://msdn.microsoft.com/en-us/library/fkx0cy6d.aspx[
^]
Click your button2, is the data loading OK now?
If you want it to autoload, you'll need to call this code when your screen loads
Other than this, I can see lots of other problems with your code!