The connection was not closed.

Don't handle with manual close(). Microsoft always recommending "using" block with connections. So always use below style.

You can see the Ref. here, https://msdn.microsoft.com/en-us/library/system.data.sqlclient.sqlconnection%28v=vs.110%29.aspx?f=255&MSPPError=-2147217396 

This style is applicable to any class implements IDisposable interface.