External Display turns black when mac is covered

I was using this MacBook Pro 2020 13”for like three years, and used it without opening the monitor and connected to a external display for almost half year but lately I opened my max monitor again and try to close it and use my external display as main display, my external display blacked out but my mac is still running with the fan turned on, have respond from keyboard and mouse but the external display just won’t show anything can anyone help me?Also when my mac monitor is turned on my external display flashing white noise I don’t know how to fix it help