It depends on where you live, because the rules of every board are different. Rules vary by state, and sometimes even by county or city. You'll have to check with your local board.
For example, in Florida if you don't renew in 2 years, your license becomes delinquent. To renew a delinquent license you don't have to re-take the national exam, but you have to submit evidence of continuing education and pay delinquent fees.