This might be related to items listed in Known Issues and Solutions, particularly, Grub gained security fixes. Updating it may be risky, or Grub - “Error: grub_is_using_legacy_shim_lock_protocol not found" may be worth looking at.
In the meantime, these might be of use;
- Basic Manjaro Linux Rescue and Recovery
- Resolve an unbootable Manjaro (black screen, login failed, cant boot, etc)
- Recovering from an interrupted update/upgrade
When possible, please provide system information as described (below).
Regards.
Welcome to the Manjaro community
As a new or infrequent forum user, please take some time to familiarise yourself with Forum requirements, and the many ways to use the forum to your benefit:
Update Announcements
The Update Announcements contain important information and a Known Issues and Solutions section that should generally be checked before posting a request for support.
System Information
While information from *-fetch type apps might be fine for someone wishing to buy your computer, for Support purposes it’s better to ask your system directly; 
Output of the inxi command (with appropriate parameters, and formatted according to forum guidelines) will generate information that may be useful for those wishing to help:
inxi --filter --verbosity=8
or the short form:
inxi -zv8c0
- inxi man pages (Arch Manual)
Be prepared to provide output from other commands whenever asked. It’s equally important to provide as much actionable information as possible in your first post, rather than simply indicating there is a problem.