We can begin with some system info;
inxi -Farz
A sort mirrors and sync and upgrade dance couldnt hurt;
sudo pacman-mirrors -f && sudo pacman -Syu
After which we may also look at foreign packages
pacman -Qmq
Of course opening nautilus should not crash things.
Have you tried disabling your extra extensions?
Not sure to what you are referring … but I doubt the system is somehow ‘reverting’ to the defunct ubuntu desktop environment.