What should I do if the computer screen goes black and No video input and Enter sleep mode appear?

The immediate priority when encountering a "No video input" message on a black screen is to systematically isolate the source of the failure, which lies within the chain of connections and components between your computer and its monitor. This message, often accompanied by a prompt to enter sleep mode, is generated by the monitor itself, indicating it is receiving power but no active signal from the computer's video output. Your actions should therefore focus on diagnosing whether the issue originates from the monitor, the connecting cable, the computer's video source, or the computer's internal state. Begin with the simplest physical checks: ensure the monitor is powered on and that the video cable—whether HDMI, DisplayPort, DVI, or VGA—is firmly seated at both the monitor and the computer. If possible, try a different, known-working cable or port on both devices, as a faulty cable is a common culprit.

If physical connections are verified, the next step is to determine if the computer is actually booting successfully despite the lack of display. Listen for auditory cues like normal startup sounds or the whirring of fans, and observe indicator lights on the computer case for activity patterns that suggest normal operation. If the computer seems to be running, the problem likely resides with the specific video output path. For desktop users, this involves checking whether the video cable is connected to the correct graphics port; if your system has a dedicated graphics card, the cable must be plugged into its outputs, not the motherboard's integrated graphics ports. For laptops with an external display, try using the built-in screen to see if the issue is isolated to the external monitor's settings or connection.

When basic troubleshooting fails, the cause may be more significant hardware or software failure within the computer. A complete lack of signal, even after a forced power cycle (holding the power button down for several seconds, unplugging the desktop from power for a minute, or removing the laptop battery if possible), points to deeper issues. This could range from a failed graphics processing unit (GPU), a loose internal component like the RAM or the dedicated graphics card in a desktop, or a corrupted system state preventing a proper boot. For desktops, opening the case to reseat the RAM and GPU can resolve connection issues, but this requires caution regarding static electricity and component handling. In such scenarios, the problem often requires professional diagnosis, especially if the system shows no signs of life or emits unusual beep codes.

The implications of this diagnostic process are practical: it moves from user-addressable external fixes to potential internal hardware failures. If the computer is under warranty, contacting technical support is advisable before extensive internal disassembly. If it is an older system and you have ruled out the monitor and cables, the cost of repair for a failed GPU or motherboard may approach the value of the machine itself, making replacement a consideration. The sequence—check external connections, verify computer state, test alternative hardware paths, and finally consider internal components—provides a logical framework to identify a fault that, while disruptive, is often resolvable without immediate professional intervention if it stems from a simple loose connection or configuration error.