A monitor can start flickering for many reasons, to fix this issue first check the connection cables and ensure that the monitor is also receiving enough power. Monitor flicker can also be caused by using a refresh rate or resolution incompatible with your monitor or when using outdated/corrupt graphics card drivers. You can test to see if the monitor itself is at fault by connecting it to a different device or port on your computer, if the flickering persists then it may be caused by a defect or internal issue. Refer to the user's manual for more troubleshooting information or reach out to the manufacturer directly for further assistance.