Although some U.S. visas, such as for tourists, are issued with expiration dates that are years into the future, others expire on shorter timelines.
A K-1 fiance visa, for example, is good for only 90 days after being issued by a U.S. consulate. Most immigrant visas, such as for spouses of U.S. citizens or employees of U.S. companies, are valid for six months after being issued.
These shorter expiration dates present challenges during the coronavirus (COVID-19) pandemic, whether you are staying home for the moment due to health and safety concerns, or you are from one of the countries from which travel is barred except for U.S. citizens, permanent residents, and a few others.
U.S. immigration authorities are still working out the details of how to deal with visa-expiration issues. You'll definitely want to keep a close eye on the date in question, however. The one thing that's certain is that you will not be allowed to enter the U.S. with an expired visa. You probably won't even be allowed to board the plane that would take you there.
The key will be to get in touch with your local U.S. consulate or embassy (the one from which you obtained the visa) well in advance of the expiration date (weeks as opposed to days).
Many, but so far not all consulates are canceling visa appointments or reducing their services in light of the pandemic, but can make exceptions for urgent situations.
It's possible that the U.S. consulate will be able to give you a new visa stamp, to substitute for the old one, with a new date that's farther into the future. This can happen only if your eligibility for the original visa remains completely unchanged, however; for instance, you still plan to marry your U.S. citizen fiance, and that person still has a job that's sufficient to support you.
If that situation doesn't describe you, you might, unfortunately have to start the entire immigration process over, with a new visa petition or application to the U.S. consulate.