If you let the registration on your domain expire, your registrar has to have an alternate plan for making money from your domain. Your readers, looking for your blog, will provide them their needed income.
If you don't renew the registration for your domain promptly, most registrars will change your domain DNS setup, and point it to a "parked domain" server. Any of your would be readers will see a screen full of ads, instead of your blog. That's your registrar, making money from your unrenewed domain.
While your domain is hosted on the parked server, it is available to anybody who will pay for it, on a first come, first served basis.
When a domain is available, it is said to be "in Redemption". Fortunately, it's not difficult to identify an expired domain.
You can rescue your domain, when it is expired (immediately), or in Redemption status (later), but there will be several complications.
- You will probably have to pay an extra fee for late registration. The extra fee is not negotiable, any more than the "bounced check" fee levied by your bank. You pay the registrar.
- You have to deal with the registrar, now. If you bought the domain through Blogger, that's in the past. All support for the domain will involve eNom or GoDaddy.
- You will have to setup the DNS addresses for the domain, on your own. If you initially purchased the domain using "Buy a domain", and had the domain setup for you, you're now going to have to learn how to setup the domain manually.
And next year, you will hopefully renew more promptly.