wtf, this video again 
i thought this was gone a long time ago but i guess not. i just left a nice friendly comment on the video's comment section and flagged it. i also left links for the correct info.
@JoshithaR , please edit post and remove the video before someone else see's it and follows it. that video has you install GDM, SDDM, and LightDM at the same time and follow the instructions for each. last year when he started spamming it in every optimus related thread here on the forum he was told to take it down by a number of people and the video's were scrubbed from the forum. the fact that over 13.000 people watched that video is just plain scary. and likely contributed to the count of people who say "manjaro broke my computer" when really the issue is a PEBCAK issue.
it's not your fault, dont take this as directed toward you. it's the person who created it and then left it up for others to follow after being corrected and asked to remove it.
did you follow every step of that video? (hope not)
the acpi_osi=! parameter can sometimes break the touchpad, it's fairly common. there are other parameters you can try instead or try removing only that parameter and see if your still able to boot. you can do this right from the grub menu.
- move selector to manjaro, dont hit enter and instead hit "E"
- find
acpi_osi=! and delete it.
- then CTRL+X to boot.
you can do this to try other parameters as well, these edits are only for that boot and are not persistent if they dont work, you would just reboot and your changes get reset to the way they are in /etc/default/grub
you can also try removing both parameters and try this one
acpi_rev_override=1
also try 2,3,4,5 separately instead of 1 if it does not work. if it does, edit /etc/default/grub accordingly and sudo update-grub so the changes become persistent.
btw, i usually tell people to just reinstall manjaro after following that video if the install is new anyhow. and if not, at least undo somee of what the video instructs to do.
)