After a short and quick analysis, I found Juniper JunOS devices may get stuck in the boot process or fail to boot the OS, in rare cases, after a sudden power loss or ungraceful power shut down. Juniper routers, switches and firewalls can experience file system corruption, which prevents the device from recovering to a functional state. It is recommended that customers minimize their log configurations to prevent excessive read/writes to the file system, which reduces the stress on storage media and reduces the potential occurrence of this issue. Moreover, if abrupt power failures are transient for a very short period of time, the availability of an UPS can also prevent the device from experiencing a sudden power loss.
You do not have to worry about damaging hardware in these situations, as the hardware cannot tell the difference between a graceful shutdown and pulling the power cord. The potential for damage is with the file system structure. It is possible for data to be corrupted, when the computer's power is interrupted with the operating system running. The data could be in the nodes, which could result in files being lost or file contents being corrupted.
Although rare, this issue more likely occurs on platforms that use a UNIX/BSD-based operating system, such as Junos, to access the flash-based storage media.
“Although rare, file system damage can occur with an abrupt power off, which may cause problems on the next boot. Use the request system halt or request system reboot command to gracefully shut down or reboot the OS. Once the OS is halted, it is safe to remove power.” - from O'Reilly Media’s JUNOS Enterprise Switching book.
There are a couple of KB discussing the fix. KB29811 is using a USB to copy a snapshot from healthy device to faulty device. KB20046 suggest to press space to go to u-boot prompt and enter some commands to fix issue.
db> help
DDB Quick Help
-------------------
Type 'c' to continue, 'reset' or 'panic' to restart.
print p examine x search set write
w delete d break dwatch watch dhwatch
hwatch step s continue c until next
match trace alltrace where bt call show
ps gdb reset kill watchdog thread panic
ddbdumpsys dumpsys
db> reset