After a domain expires, it will be available for registration by anybody. It is recommended to renew your domain as early as 90 days in advance in order to prevent a disruption in service and possible loss of your domain name.
In the rare event that you do forget to renew your domain name and someone else renews it, there is very little you can do to recover it. In some cases, the new registrant may be willing to sell the domain.
Alternatives to buying back a domain are to register the same under with a different TLD (www.yourDomain.net instead of www.yourDomain.com)