A virtual Linux machine used for development was behaving unreliably.
Programs would fail to start and running program would freeze.
I decided to restart the system, however it failed to start up again.
It simply displayed this message:
BusyBox v1.22.1 (Ubuntu 1:1.22.0-15ubuntu1) built-in shell (ash) Enter 'help' for a list of built-in commands. (initramfs)
I tried to exit the shell, which lead to this message:
/dev/sda1 contains a file system with errors, check forced.
/dev/sda1:
Inodes that were part of a corrupted orphan linked list found.
/dev/sda1: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.
(i.e., without -a or -p options)
fsck exited with status code 4
The root filesystem on /dev/sda1 requires a manual fsck
File system corruption seemed to explain the reliability problems and the subsequent boot failure.
Followed the suggestion of running fsck with:
fsck /dev/sda1
Accepted all file system repairs suggested by fsck.
(Should have used the -y parameter)
Then rebooted the system with:
reboot
After the file system repairs the system booted and seemed fully functional.
Conclusion
If a Linux system fails to boot and only displays a BusyBox / initramfs prompt, try exiting the shell.
This may provide information about the actual problem.