If you have previously entered the United States but your visa is expired, then you need to re-apply for your visa prior to seeking re-entry to the United States.
If you stay in the United States beyond the expiration date of your visa, then you will be considered a visa overstay. You may face severe penalties – including being banned from the United States for 3 to 10 years (depending on the length of your overstay).
If you try to enter the United States with an expired visa, then the CBP officer will refuse entry and you will need to return to your home country. In your home country, you can apply for a new visa or request a visa extension.