Nothing happens if your USA visa expires while you are still in the country.
Unlike most countries, the USA visa expiration is not related to how long you can stay in the country.
Instead, the visa expiration indicates the last day you can enter the USA. Even if you had entered on the visa expiration day, you could receive a normal six months admission.
The date you must depart the USA is the date stamped into your passport when you entered the USA. If you cannot find the stamp or want to confirm the date, you can look at your online I-94 record, which should also show the date you must depart.
I got suspicious and went to read your previous questions. They revealed that you have entered the US once before and were admitted until October 8, 2018, and that you got an admission stamp which was stamped partly across your visa. This is just a regular admission stamp for your visit. On your first visit using a visa, CBP will place the admission stamp so that it is partly on the visa. This does not cause the visa to expire. The visa expiration date remains as it was originally printed. It simply means your initial trip to the USA must end by October 8. But you already left the USA, so it is no longer relevant.
Here we see that this visitor's first visit to the US using this visa was in January 26, 2013, and she was admitted unitl July 25, 2013. But the visa remains valid for multiple entries and several more years.
Admission stamps you get for future visits will be placed elsewhere in your passport.