See that, it’s important for this time:
But usually you should just do:
yay
Or separately upstream and than AUR packages if you prefer, like that:
sudo pacman -Syu
yay -Sua
If there are are no errors in the terminal output - you should be fine 
If there were Kernel / video drivers update - you should reboot (but 
should notify you about that both in terminal and system notification)
Usually just go to TTY and try to update again / see if there are some errors and you can fix them.
If that fails and you can’t boot, go to live usb and chroot, then try to update / restore Timeshift, see bottom of that page:
https://discovery.endeavouros.com/applications/update-troubles-meet-timeshift/2019/11/