Gfilui

I have installed/updated windows while Ubuntu was installed in my PC. As a matter of fact, grub is gone. I also want that, when my PC is booted, it directly boots into windows. And I don't want the grub screen.

But I want to occasionally log into Ubuntu for learning and some other purposes.

What my query is, Is there any option to log into Ubuntu (that is still installed in my machine) without restoring Grub. Because I don't want others to know that Ubuntu is installed in my PC.

You must have Grub to dual-boot Ubuntu, but there are some other options that may fit your needs.

Answering your question:
The time that the grub boot screen is displayed can be modified, so you
can set the time to zero. Then you would have to have Windows set as your default OS and use the information on this forum for when you wish to select your Ubuntu OS. The information shared there includes some hackery, so I won't go into detail describing it here.

Option 2:
Using a bootable flash drive might meet your needs. This requires you to change the boot order in the BIOS settings, but it is quite straightforward and is perfect for random tinkering. There is also much less of a chance of overwriting partitions, so that's a huge plus. Flash drives are cheap, so for $15 you could try out a half dozen different distros.

Option 3:
Use virtual machines to run on your computer. This is free using Oracle VirtualBox.

Option 4: Use WSL, Windows Subsystem for Linux. This is a new project that allows a Windows user to experience most of the features of Linux directly on their Windows machine.

Option 5: Get a Raspberry Pi Zero and ssh into it via the serial port. This is awesome for a true Linux environment on the go.

Note: I personally use all of these methods. Each has its advantages and disadvantages.

After surfing many places at last I could get a work around. The links given in the answer of Brett Holmann above helped me little. Finally I could successfully hide the grub menu/screen applying the solution given by user310707.

At first I set the default Boot into Windows, then hidden the Grub screen. So now my PC always directly boot into windows.

But (as said in some posts), SHIFT key does not bring back the GRUB Menu/screen. So if I need to log into my Ubuntu (Lubuntu), I use a tool which lists all available/existing OSs in the machine and enable to boot into any one of those. Tho name of the tool is Super Grub2 Disk.

So, using that I can occasionally log into Ubuntu while that is hidden in normal boot time. Exactly what I wanted...