In short: When a domain expires, the website and email stop working. You usually have a grace period of about 30 days to renew at the normal price, then a costlier redemption period, then the name is released.
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What happens, in order
On the expiry date the site and email usually stop, and the registrar may show a parking page.
A grace period follows, often around 30 days, when you can still renew at the normal price. After that comes a redemption period with extra fees. Finally the domain is released and anyone can register it. Exact periods differ by registrar and extension.
How to check your expiry date
Use the free Kiplock domain check to see the date and registrar, or look in your registrar account.
How to avoid losing a domain
Turn on auto-renew and keep a valid payment card on file. Use an email address for the registrar account that you will always read. Register for several years if you can. Set a reminder a month before the date.
If it already expired
Log in to your registrar and renew straight away. If it is in redemption, contact support and ask for the redemption fee. Once released, you may have to buy it back from whoever registered it.
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Check your domain expiryCommon questions
Can I get an expired domain back?
Usually yes during the grace and redemption periods. After release you would have to buy it from its new owner.
Will my email stop too?
Yes. Email addresses on the domain stop working when the domain stops resolving.
Does auto-renew always work?
Only if the card on file is valid. Check it each year.