Brightness control got broken in Ubuntu 20.04 on Lenovo 5i

[musamaanjum]

I'd installed Ubuntu 20.04 by using the method given in this repo. It works completely. The graphics drivers are installed and brightness control keys also work.

After several months, a disaster has happened. Somehow the brightness isn't being controlled by buttons or from inside the panel at all. I've tried debugging it. But I'm unable to reach anywhere. Any ideas and help will be much appreciated that how can I find the root cause and debug the issue.

sudo lshw -C display
  *-display                 
       description: VGA compatible controller
       product: TU116M [GeForce GTX 1660 Ti Mobile]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:01:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress vga_controller bus_master cap_list rom
       configuration: driver=nvidia latency=0

[musamaanjum]

Fortunately, I can change brightness with xbacklight commands. I'm okay with buttons or menu not changing the brightness.