It could be any number of problems and this web page should help narrow it down. Also what is the exact model number of your system?

Hi @HotKar  Community Manager says: All G Series Laptops questions should be posted HERE  on the Inspiron Laptops forum board. Many users say it would be more logical for users to have: 

Dell Community > Laptops > Dell G  

Grouping similar laptops together allows user to user discussion to focus on a particular system Series and Subject. 

Have you tried reinstalling windows? and also installing all the drivers for the laptop? Because I had an Issue with my desktop where the Button was fine but windows was having an issue where it would just not sleep after a while even with a hard power reset using the button so I had to shut down manually.

I have checked all devices in device manager, I couldn't found missing driver or failed device there.

Also I run Dell Support Assist, it reported that all driver is up-to-date.

So have you installed windows on another drive and the power button doesn’t work.

Hi @HotKar  have you visited Windows 10 Control Panel\All Control Panel Items\Power Options\System Settings and looked at what the power button does? Options for "When I press the Power button" are: 

1. Do Nothing: When you push the power button, nothing happens.
2. Sleep: Your PC goes into a low-power sleep mode but stays running.
3. Hibernate: Your PC saves the state of your system memory on disk and shuts down. Your session can be resumed later when you power back up.
4. Shut Down: This shuts down Windows and powers off your PC. (This is the default.)
5. Turn off the Display: Your display turns off, but your PC keeps running.

The Power button options can be different for On battery or Plugged in. When on ac adapter power, does Dell G5 power button work? Check to see if an application, game, etc. is changing these settings for you. 

Please click on Kudos to say thank you for response from user that is not employed by Dell. Please share an update on progress, so that other users derive benefit from your experience. Thank you. 

Hi, @crimsom , all of these options are set to "Sleep", but I have tried all the options (hibernate, power off) already.

It doesn't do anything regardless of plugging into mains or using battery. 

But:

I made an image backup, and I did a clean install of Win 10. All the functions of power button works properly.

I'm afraid something has got wrong in the procedure of preinstalling Windows 10.

Unfortunately this PC is my "toolbox", full of specific programs, so I had to restore image in order to do work. If I have a lot of time I will reinstall operating system and programs.

Hi @HotKar  thank you for sharing update. 

Agree, not sure why the Windows 10 x64 operating system behaviour has changed on your current OS(C:) boot drive. Having an image of your current OS(C:) boot drive is an excellent option within your disaster Recovery Plan. 

You could get a M.2 2280 NVMe drive installed with windows, and in slow time swap in this alternative OS(C:) boot drive to be prepared with Windows and Dell updates. Then check to see if power-on button has the expected default functions. This fully functional alternative OS(C:) drive would then be your 'gold' standard disaster recovery drive. Then use 'gold' OS(C:) boot drive to clone a 'silver' OS(C:) boot drive. Then incrementally add your "toolbox" of applications to update the silver OS(C:) boot drive, and hopefully get a fully functional drive.