You can enable an existing constraint using the ENABLE CONSTRAINT clause of ALTER TABLE.
To enable a constraint, all the rows in the table must satisfy the constraint.
SQL> -- create demo table
SQL> create table myTable(
2 id NUMBER(2),
3 value NUMBER(6,2)
4 )
5 /
Table created.
SQL>
SQL> ALTER TABLE myTable
2 ADD CONSTRAINT uq UNIQUE (id) DISABLE;
Table altered.
SQL>
SQL> ALTER TABLE myTable
2 DISABLE CONSTRAINT uq;
Table altered.
SQL>
SQL> ALTER TABLE myTable
2 ENABLE CONSTRAINT uq;
Table altered.
SQL>
SQL> drop table myTable
2 /
Table dropped.
SQL>