Hello.

I successfully upgraded my laptop to Debian 11 but for some reason suspension has stopped working.
The screen turns black, as if switching off, but the computer is still running (the fans and everything seem going on).
The problem is I can't make it come back and I have to hard reset the computer.

Do you have any idea why this could be happening now?

Thanks.

This is most likely a Debian issue not related to KDE, particularly as it is after a major upgrade.
You might want to inquire on a distro-specific venue.