I have a problem running my PL/SQL script in SQL*Plus. I can run SQL commands normally but when I want to a run any PL/SQL code it gives nothing. See code and output below.

DECLARE
x_salary employee.salary%TYPE;
BEGIN
select salary
into x_salary
from employee
where ssn=&enter_ssn;
--Output the result
DBMS_OUTPUT.PUT_LINE('Salary is ' || x_salary);
EXCEPTION
--Output when no records are returned
WHEN no_data_found THEN
DBMS_OUTPUT.PUT_LINE ('No employee found');
WHEN others THEN
DBMS_OUTPUT.PUT_LINE ('Error encountered, but cause unknown');
END;