If you applied for and were granted a temporary ("nonimmigrant") visa to come to the United States before the coronavirus (COVID-19) epidemic began, but you have not yet entered the country, your next step depends largely on:
We'll look further into these concerns below.
In an attempt to limit spread of COVID-19, the Trump Administration has flat-out blocked travel to the U.S. from various countries, with only limited exceptions. See the State Department's summary of relevant Presidential proclamations and other information on which countries are affected.
Obviously, if you are from one of the blocked countries, and you do not fall into an exception (which are mostly for people whose entry would be in the U.S. national interest or serve its law enforcement objectives), you should not consider U.S. entry at this time.
Besides this issue, you will likely find it difficult to find transportation to the U.S., as many airlines are grounding much of their fleet.
If you received a visitor (B-2) visa for holiday travel to the United States, or you come from a Visa Waiver country, you'll probably need to put your plans on hold, even if your entry would be permitted.
Remember that even if you're allowed to board a plane or other transport, you will be screened upon entering the United States. Any sign of COVID-19 symptoms could result in you spending your vacation in quarantine.
If your stay was meant to be a longer one, perhaps as an H-1B worker or F-1 student, your future depends in part on who petitioned you. The job must still be in existence, and the school must remain open, in order for a visa to remain valid either for entry or after arrival in the United States.
Unfortunately, this pandemic has led to the closure of many businesses and schools.
Visas for tourists are often issued with expiration dates years into the future, so at least you might be able to use it at a later time. And employment-based visas usually don't expire for a few years. The H-1B visa, for instance, is good for three years.
Some visas, however, expire on shorter timelines. A K-1 fiance visa, for example, is good for only 90 days.
U.S. immigration authorities are still working out the details of how to deal with visa-expiration issues, but you'll want to keep a close eye on the matter. Obviously, you cannot enter the U.S. with an expired visa.
If your eligibility for the original visa remains completely unchanged (for instance, you still plan to marry your U.S. citizen fiance, who still has a job that's sufficient to support you), but your U.S. entry visa will soon run out, you might be in luck.
Contact the U.S. consulate and embassy that issued the visa, and ask for a visa stamp with a new date.
If that situation doesn't describe you, you might, unfortunately have to start the entire process over, with a new visa petition or application to the U.S. consulate.