This is a rather scary prospect and it can happen if your email address at the domain registrar is not kept up-to-date.
Often when people first register a domain name they use a personal address or an address from their ISP account. If they don't check that account for mail anymore or they change ISP's and the address is deleted, they will not receive renewal notices and could lose their domain.
It's important to check that your contact details for your domain are current! You can check using this tool at http://network-tools.com/default.asp?prog=whois