I've got the same with a 2020 MBP M1 and Samsung external monitor. I've fixed it by unplugging all cables from Mac and rebooting screen and Mac. By which i mean... not just turn off the screen with the button on the screen, but really pull the plug out of the wall. then reboot mac, then plug in screen power, let screen come on and plug into Mac. Some posters on other sites are saying that you have to use mac power supply for the mac... but I always have that and my external screen is also plugged into mains power. Good luck...