In Ubuntu, the graphical boot animation is handled by Plymouth. You can easily change its appearance by switching to another Plymouth theme. After a lot of changes and edits, you can also return to the default Ubuntu boot screen animation by using the below method.
Restore Ubuntu’s Default Boot Screen Animation:
In the terminal application (ctrl+alt+t) run below command
sudo update-alternatives --config default.plymouth
Now select “bgrt.plymouth” boot animation and type the corresponding selection entry number and hit Enter.
In my case the “type selection number” is 1.
After that run below command.
sudo update-initramfs -u
It will take some time. After complete restart your system, your boot animation will be back to normal.
If you completely removed plymouth-theme-spinner then, install it using below command.
sudo apt-get install --reinstall plymouth-theme-spinner