Filesystem failing to mount on boot, but OK when mounted manually
Solution 1:
Without the fstab file I cannot say much, but if it fails to mount a disk it will go into emergency mode.
You can prevent this by adding the nofail option. This will cause the disk NOT to be mounted and continue booting anyway if an error occurs while mounting.
A common cause of this error is referencing a disk as /dev/sdX. If there are other disks connected it may cause to try to mount another disk, and could fail due to specific options.
I may be able to provide more details if you provide an fstab file.