A new desktop computer has recently been built and Debian 11 installed. All the hardware is new including monitor, keyboard
and mouse.

For the last couple of mornings, when coming to use the computer, the monitor was found to display a 'no signal' message, although
the computer was switched on okay. On the first morning, the monitor suddenly came to life and worked okay the rest of the day.
On the second morning, the monitor again showed 'no signal' and this time both the keyboard and mouse would not
work with the monitor.

Later the monitor sprang into life after the mouse was moved. Both keyboard and mouse worked. I am worried in case monitor
again gives 'no signal' message.

Could the problem be due to a bug in Debian 11 rather than in the hardware, given that all the hardware is new ?

This looks like a DOA hardware component. Check monitor cable first. Which type of connectors are used between compouter and monitor? HDMI and DP connectors may be baggy stuff.

From your post I am guessing you are letting the computer sleep/hibernate versus poweroff? Then using the mouse or keyboard to wake it up?

If you shutdown / restart the computer does everything work as expected?

I'm fairly sure any software problem wouldn't say "no signal", it'd just give a blank screen or similar.

Does sounds like a cable problem, (though could also be the port inside either computer or monitor).

Unplug and replug both ends of the cable to make sure they're both fully seated, then see if wiggling the cable causes signal to drop/restore.

If you've multiple ports, try the cable in each of them; if you've got another cable try that to see if that makes a difference; if possible test if the potentially faulty cable exhibits the same issue on different hardware.

IME, some displays with multiple input types do not play reliably with PCs that boot from UEFI firmware. If problem remains with other cables of same type, and you have the option to switch to/from HDMI<>DisplayPort with a different cable, give it a try. If the problem disappears with one cable type, check for a display firmware update. If none is available that fixes it, return the display while you can and get another, using warranty if necessary.

I usually see the 'no signal' message when linux shuts down, sleeps, lock screen, etc. and the monitor is still powered on. The monitor then usually goes to standby after some time dealy. Usually the monitor then wakes up again when the PC sends a signal to the monitor.

This also happens when the monitor has been powered off and the PC is also off, asleep, etc. but the monitor is powered on before the PC is awake. The monitor checks for a signal and if none then it gives the 'no signal' message.

Maybe this is related to timing and not a hardware or software issue at all.

@debianlearner the motherboard has failed. I recommend you replace it with a Gigabyte motherboard.

The system is working again okay.

I now don't know how the problem started or ended.

This is of concern because the problem might suddenly return.

Could still be a cable issue - did you try any of the suggestions?

Better to chase down an issue when it's not urgent than to wait for it to re-occur at an inconvenient time.

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