Although some U.S. visas, such as for tourist visitors (B-2), 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.
You'll definitely want to keep a close eye on the date in question, since 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 consulates have reduced their hours or services in light of the pandemic, but can make exceptions for urgent situations.
For an immigrant visa, 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.
For nonimmigrant visas, the U.S. consulates have expanded the circumstances under which you can ask for a "waiver" of the interview requirement, meaning you'd apply for and receive your renewal without actually visiting the consulate. Specifically, they're most likely to grant an interview waiver when:
If nothing else works, you might, unfortunately have to start the entire immigration process over, with a new visa petition or application to the U.S. consulate.