Please try the suggestions below:
1. Please check if you can still discover your primary machine while still in black screen. If yes, can you please try to reconnect the spacedesk viewer and check if it will reattach you main monitor display.
2. Try to blindly attach the main display using display switch. Just press the Windows logo key + P hotkey maybe a few times to try to switch to desktop extension or duplication. For assistance, please check the “Configuration and Settings > Windows Driver (Primary Machine) > Desktop Extension or Desktop Duplication (mirror) settings in Display Switch” chapter in the user manual – https://forum.spacedesk.netuser-manual/#item_471