For some reason, in Ubuntu 20.04, with every kernel upgrade, the system loses its dual-monitor configuration and only starts to work with one monitor due to lack of simultaneous upgrade of linux-modules-nvidia-390XXXX (in my case).
The reason is that during each Ubuntu kernel upgrade, the kernel for NVIDIA drive module for that new kernel is not upgraded automatically and it should be done manually.
In my case, my Ubuntu 20.04 workstation was upgraded from linux-modules-5.4.0-39-generic to linux-modules-5.4.0-40-generic. After the upgrade, I lost my dual monitor configuration.
To fix it I installed linux-modules-nvidia-390-5.4.0-40-generic (new kernel) and rebooted the system. Done!
Dual monitor mode restored:
$ nvidia-smiThu Jul 2 18:35:01 2020 +-----------------------------------------------------------------------------+| NVIDIA-SMI 390.138 Driver Version: 390.138 ||-------------------------------+----------------------+----------------------+| GPU Name Persistence-M| Bus-Id Disp.A | Volatile Uncorr. ECC || Fan Temp Perf Pwr:Usage/Cap| Memory-Usage | GPU-Util Compute M. ||===============================+======================+======================|| 0 GeForce GTX 550 Ti Off | 00000000:04:00.0 N/A | N/A || 41% 43C P0 N/A / N/A | 344MiB / 957MiB | N/A Default |+-------------------------------+----------------------+----------------------++-----------------------------------------------------------------------------+| Processes: GPU Memory || GPU PID Type Process name Usage ||=============================================================================|| 0 Not Supported |+-----------------------------------------------------------------------------+