Welcome to Manjaro! 
- Please read this:
How to provide good information
and post some more information so we can see what’s really going on. Now we know the symptom of the disease, but we need some more probing to know where the origin lies…
- An
inxi --admin --verbosity=7 --filter --no-host --widthwould be the minimum required information for us to be able to help you. (Personally Identifiable Information like serial numbers and MAC addresses will be filtered out by the above command)
Also, please copy-paste that output in-between 3 backticks ``` at the beginning and end of the code/text. - Are you rebooting into Windows when this happens or straight Manjaro reboots? If the former: Please don’t warm boot from Windows into Manjaro: shut down Windows completely and then press the power button to cold boot into Manjaro because some Windows hardware drivers mess up the UEFI system and solve that for themselves by resetting the UEFI system whereas Linux (and thus Manjaro) expects a “clean” UEFI system.

P.S. If you enter a bit more details in your profile, we can also see which Desktop Environment you’re using, which CPU/GPU or Kernel, … you have without typing it every time