boot – Ubuntu 18.04 Black Screen after restart – Need urgent help

What happened:

I restarted my desktop computer. But, when the computer reboots, the display shows a complete black screen (there is no mouse. The monitor does receive signal from HDMI input), and nothing shows up for a long time.

Therefore, I wanted to shutdown the computer. I pressed the power button lightly for a few seconds. The ubuntu logo with a few dots shows up. The color of the dots changed,for a few seconds. Then the computer shut down.

Then, I pressed the power button to boot the computer. No Dell logo screen shows up (that is, the screen that prompts F12 or DEL for boot options does not show up). Complete black screen again. I repeated the above, lightly pressed the power button, and the ubuntu logo screen would show up before the compute shuts down.

I repeated the above for several times. Always black screen. I was not able to log in, of course.

To believe this is an Ubuntu issue, what I have tried:

One time, after pressing the power button, I repeatedly pressed DEL, hoping the screen of Boot Options would show up. It did not. However, then, I pressed the Down arrow button, for two times, and then hit Enter. Then, the computer boots into Windows (I have a dual boot of Windows 10 and Ubuntu 18.04). Windows behaves completely normally. I repeated this procedure one more time, and I was able to get into Windows, without looking at the menu screen in Boot Options.

I believe something happened with Ubuntu, which made both the first boot up screen blank, and the UEFI Boot Option selection screen blank. If I don’t blindly enter the Boot Options screen with DEL, the computer boots into Ubuntu by default, and the blank screen continues, until I lightly press the power button which causes a brief display of ubuntu logo before shutdown, as described above.

What happened before I first restarted:

I remember two things: First, I did a sudo apt-get update command. I didn’t do upgrade following it. I attempted to download and install Steam, following this link, using the wget command to download the .deb file and then install it. The installation was not successful. Therefore I wanted to switch to windows, by rebooting. And then the whole things started.

More info
I have a Dell XPS Tower 8930. I previously had a dual boot of Ubuntu 16.04 and Windows 10. Lately I upgraded the Ubuntu to 18.04.

Please help!