DIAGNOSIS A single pixel that is permanently off (black) on a dark background is a dead sub-pixel. This is a physical manufacturing flaw, not something you can fix with software. Most manufacturers have a minimum defect policy for returns.
FIX Check Sansui's official dead pixel policy for your region. Most budget monitor brands require a significant number of dead pixels (often 3-5 or more in a central cluster) for a warranty return. If it's just one, they will likely deny an RMA. Your first step is to contact the retailer you bought it from (Amazon, Newegg, etc.), as their return window (usually 30 days) is more lenient than the manufacturer's warranty.
IF THAT DIDN'T WORK You can try gentle, localized pressure. With the monitor OFF, place a soft, slightly damp microfiber cloth over the pixel. Using the rounded end of a pen or a pixel-fixer tool, apply very gentle, circular pressure for 5-10 seconds. Turn the monitor on to check. This can sometimes re-establish a connection but risks causing more damage.
HEADS UP Do not use online "dead pixel fixer" flashing videos. Those are for stuck pixels (red, green, blue), which are different. Flashing colors will not revive a physically dead pixel. Also, avoid pressing directly on the screen with a sharp object, as you can easily damage the VA panel and create a permanent white spot or crack.