When it comes to managing your website you need to ensure that your domains are always renewed on time. Most domain name providers give you ample notice of when your domain is due to be renewed and are often also offer you the option to automatically renew your domain.
Even if you do forget and your domain name expires you would normally get a grace as period of about 60 days to allow you in which to renew the domain before anybody else can purchase it. If you do forget to renew your name and the 60 days passes then the domain will become available to the public again which may mean you may lose it.
If you domain name is purchased by somebody else then you have no right to claim it back. Once your domain has expired your website will go offline. If this happens then you will have to purchase a new domain name and change your website a point across to it, it may also affect your email address and may mean that you have to have stationery reprinted, such as business cards and letterheads etc. Easiest way to manage your domains is to set them on automatic renewal and if you wish to cancel go in and manually cancel the domain.