Error connecting a program with an Oracle database

I'm trying to connect a Java program with a database made in SQLPlus Oracle 11g, at first the connection worked but then I got the following error:

java.sql.SQLRecoverableException: Error de E/S: The Network Adapter could not establish the connection
    at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:489)
    at oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:553)
    at oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:254)
    at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
    at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:528)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:678)
    at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:229)
    at connection.ConexionOracle.conectar(ConexionOracle.java:31)
    at connection.ConexionOracle.consult(ConexionOracle.java:73)
    at connection.ConexionOracle.getColumn(ConexionOracle.java:60)
    at view.JpanelCreate.<init>(JpanelCreate.java:35)
    at view.JpanelCRUD.<init>(JpanelCRUD.java:28)
    at view.JFrameMainWindow.<init>(JFrameMainWindow.java:18)
    at controller.Controller.<init>(Controller.java:13)
    at test.Runner.main(Runner.java:7)

The code where I connect the program with the database is the following:

public ConexionOracle() {
    user = "usuario";
    pass = "123";
}    

public ConexionOracle conectar() {
        try {
            Class.forName("oracle.jdbc.OracleDriver");
            String BaseDeDatos = "jdbc:oracle:thin:@localhost:1521:xe";
            conexion = DriverManager.getConnection(BaseDeDatos, user,pass);            
            if (conexion != null) {
                System.out.println("Conexion exitosa!");
            } else {
                System.out.println("Conexion fallida!");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }        
        return this;
    }

What could be failing? Is the flaw in the program, in Oracle or in another external thing?

When reviewing the files listener.ora and tnsnames.ora this appears:

tnsnames.ora

    # tnsnames.ora Network Configuration File:

XE =
  (DESCRIPTION =
    (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    (CONNECT_DATA =
      (SERVER = DEDICATED)
      (SERVICE_NAME = XE)
      (SID = XE)
    )
  )

EXTPROC_CONNECTION_DATA =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
    )
    (CONNECT_DATA =
      (SID = PLSExtProc)
      (PRESENTATION = RO)
    )
  )

listener.ora

# listener.ora Network Configuration File:

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (SID_NAME = PLSExtProc)
      (ORACLE_HOME = /u01/app/oracle/product/11.2.0/xe)
      (PROGRAM = extproc)
    )
  )

LISTENER =
  (DESCRIPTION_LIST =
    (DESCRIPTION =
      (ADDRESS = (PROTOCOL = IPC)(KEY = EXTPROC_FOR_XE))
      (ADDRESS = (PROTOCOL = TCP)(HOST = localhost)(PORT = 1521))
    )
  )

DEFAULT_SERVICE_LISTENER = (XE)

update

After reviewing several forums and trying several things, the problem became the following:

java.sql.SQLException: Listener refused the connection with the following error:
ORA-12505, TNS:listener does not currently know of SID given in connect descriptor

at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:489)
at oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:553)
at oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:254)
at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:528)
at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:678)
at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:229