If you're using a separate graphics card, make sure the video cable (such as HDMI, DisplayPort, or VGA) is securely connected to both the graphics card and the monitor. If possible, try using a different cable or connecting to a different monitor to rule out any cable or monitor issues. You can try resetting the BIOS settings to their default values by clearing the CMOS (Complementary Metal-Oxide-Semiconductor). To do this, refer to your computer's manual for specific instructions, as the process can vary depending on the motherboard model.