Monitor will not go to sleep

I’m running a ZONTAC NVIDIA GeForce 9800GT ECO GPU board and Unbintu 18.04 ( up to date)
The drivers are NVIDIA binary driver-version 340.107.
Until today, I could shut the PC down and the monitor shuts off ( black screen) and stays thay way until I re-boot the PC.
Now, the PC shuts down and the monitor shows a floating " Check cable connection" as if I have the monitor disconnected from the PC with the monitor power on.
The only way to stop this is to shut the monitor off. The same error also happens when the computer goes to sleep in an hour.
This has never happened before today. The cable is tight at both ends. Card is seated properly and screwed in place. Using 2nd HDML rconnection does not help.

“Nvidia binary driver-version 340.107 from nvidia-340 ( proprietary,tested)
" this device is using the recommended drivers”

Samsung SyncMaster 204b monitor

HDML cable- is new.

Someone else was writing about " X configuration option "HardDPMS " and this corrected the problem.
I have no clue what that is.
Please help.

The 340.107 driver is legacy and hasn’t been changed for 7 months now so it’s more likely that either a hardware failue is imminent or a kernel change is responsible.

You can also check the log for recent updates in /var/log/apt/history.log in case something has been updated and broke the previous behavior.