The internet sure makes it possible to do things quickly, sometimes so quickly it will make your head spin. One day your site is selling lots of product or is very popular, and next no one is visiting. Perhaps your site got kicked out of a major search engine or some competitor hijacked your page and your listing. Or perhaps someone just brought a new site online which is simply gaining all of your traffic.
Or perhaps you just forgot to renew your domain name. Maybe you changed your email address and forgot to update your domains contact information. Or your spam filters automatically deleted that renewal message that you got. Maybe you deleted it yourself, thinking it was just another piece of junk email. It could simply be that your domain registrar had a glitch in their reminder software, meaning you didn't get reminder email.
Whatever happened, your domain name expired. You have to understand that domains are only leased or rented for a certain period of time. You do not own them forever, and you must renew them in advance of their expiration - or you risk losing them forever.
You mentally kick yourself, load your browser and enter URL for your registrar's site. After you log in, you enter information to renew. If you are lucky, they take your money and your name is reactivated within a few days. All is well, just remember to renew beforehand next time.
Quite often, however, you will not be able to renew your domain because in meantime someone else purchased it out from under your nose. This most often happens if your name is very desirable (likely to be typed by someone) or has very high traffic, but it can occur with any domain name.
Why did this happen? Oftentimes, someone just happened to buy it for honest reasons. More often, however, there are shady motivations behind purchase.
Someone may have sniped your domain.
This simply means they've been watching your domain name, waiting for it to expire. They waited through hold period until name became available, then quickly purchased domain second it was deleted from internet databases.
These people generally have no intention of creating a web site, selling a product or doing anything you'd normally expected with a domain name. What they want to do is sell name back to you, or to anyone else who is interested.