After power-on’s, or reboots the system fails to boot, getting stuck on a black blank screen. However after a force shutdown, Manjaro usually boots on the 2nd (sometimes 3rd, 4th or 5th) time.
When it boots into black screen, try to switch to console (Ctrl+Alt+F2). Then get the /var/log/Xorg.0.log and post it here.
Another option to try is:
to boot into TTY without trying to load the DE by going to the the grub menu and editing the entry pressing e then replace quiet with systemd.unit=multi-user.target. Then press F10 to start booting.
Once you are booted and logged into TTY, start the xserver and DE manually using the startx command.