If you entered the United States on a valid visa and, before reaching the date when your permitted stay runs out, filed an application with U.S. Citizenship and Immigration Services (USCIS) for an extension or a change in your status, you may be wondering what happens if you don't hear back from USCIS before your permitted stay under your visa expires.
This is not an uncommon situation. USCIS is famous for delays and backlogs of applications.
By the way, we're assuming you actually got the application in on time. If you didn't, then your best option is to leave the United States and either get a new visa for reentry or, if you haven't yet reached the expiration date on your old visa and multiple entries are allowed on it, simply return.
We're also assuming you've correctly identified your required departure date. It's usually shown on the I-94 Arrival/Departure Record, which is different than the date on the original visa. (The latter date just shows the last day upon which you could have entered the U.S. using that visa.)
If you see that the expiration date is coming, and you're still waiting for an answer, it's a good idea to look into what's going on. You can check the status of your application online, at USCIS's case status page. If for some reason you're unable to do that, or the answer isn't helpful, visit a local USCIS office (after making an appointment using the INFOPASS system). Perhaps the officer there can give you some news. It's possible, for example, that USCIS already sent you a decision by mail, but for some reason you didn't receive it.
Once the expiration date of your permitted stay has passed, you have no actual immigration status. If you were working, based on having a visa that permitted work, you must now stop working.
On the other hand, you're not expected to leave the United States. Nor are you accruing what's called "unlawful status." This is important, because people who have accrued more than 180 days' unlawful status must, upon leaving the United States, be penalized with a bar to reentry of three years. If you accrue more than one year's unlawful status, the bar becomes ten years.
If your visa has run out and you're not getting satisfactory answers from USCIS, now would be an excellent time to consult an immigration lawyer. Don't wait until you're at risk of accruing unlawful status or otherwise jeopardizing your right to remain in the United States.