If it's the day of your wedding (or a few days before) and you've realized you've lost your certificate, head back to the registrar's office/city hall where you first picked it up and file for an emergency duplicate license. Don't sweat it, this happens all the time!
If you're already married and lost your certificate, here's what you can do.
Most countries/states/provinces allow you to reapply for your marriage license online, by mail, fax (do people still use these?) in person at a government office, or by phone. Simple, right?
To remedy the "I lost my marriage certificate!" problem you're facing, you will need a few pieces of information.
First and last name of both you and your spouse
Your wedding date
The name of the town/city you got married in
The easiest way to get the certificate is through an online application form, but no matter which route you take, you will have to pay a fee.
There are plenty of reasons why you may need another copy of your marriage certificate. Perhaps you require it as evidence of a change of name, for divorce proceedings, as part of genealogical history, to settle issues of legitimacy of a child, and the list goes on!
Either way, losing your marriage certificate can be a stressful experience (this article about how to apply for a marriage certificate can help you navigate the sometimes anxiety-inducing experience!).