For Close the wallet:

SQL> ADMINISTER KEY MANAGEMENT SET KEYSTORE CLOSE IDENTIFIED BY MyWalletPass;
keystore altered.

SQL> select * from sensitive_data ;
select * from sensitive_data
              *
ERROR at line 1:
ORA-28365: wallet is not open

For Open the wallet:

SQL> ADMINISTER KEY MANAGEMENT SET KEYSTORE OPEN IDENTIFIED BY MyWalletPass;
keystore altered.

SQL> select * from sensitive_data ;
no rows selected