Sometimes (hopefully not in production) you have "low" sequence values but "high" identifiers in the respective table. That usually results in primary key violation on an attempt to insert a new row.
So - if you really do not want to delete the data, you should make the sequence have those "high" values, ideally max(id_from_the_table) + 1.
In PostgreSQL it turns out to be much more elegant than in Oracle, where you can either come up with some PL/SQL FOR loop or use DDL ALTER SEQUENCE statements. The examples have been copied from
http://www.postgresql.org/docs/current/static/functions-sequence.html
SELECT setval('foo', 42); Next nextval will return 43
SELECT setval('foo', 42, true); Same as above
SELECT setval('foo', 42, false); Next nextval will return 42
