First that "Tech Guy" didn't knew a thing or is lying to you if he never saw a message like that. You can even Google it and you'll find dozens of articles about this, even your exact same issue.
The message you saw was a notice from the system that an error on the disk was detected, so it wanted you to run a command manually RUN fsck MANUALLY
A fragment from fsck's manual
DESCRIPTION
fsck is used to check and optionally repair one or more Linux filesystems. filesys can be a device name (e.g.
/dev/hdc1, /dev/sdb2), a mount point (e.g. /, /usr, /home), or an ext2 label or UUID specifier (e.g.
UUID=8868abf6-88c5-4a83-98b8-bfc24057f7bd or LABEL=root). Normally, the fsck program will try to handle filesystems on
different physical disk drives in parallel to reduce the total amount of time needed to check all of them.
If no filesystems are specified on the command line, and the -A option is not specified, fsck will default to checking
filesystems in /etc/fstab serially. This is equivalent to the -As options.
The exit code returned by fsck is the sum of the following conditions:
0 No errors
1 Filesystem errors corrected
2 System should be rebooted
4 Filesystem errors left uncorrected
8 Operational error
16 Usage or syntax error
32 Checking canceled by user request
128 Shared-library error
Your exit code was 4, that meant Filesystem errors left uncorrected and was on /dev/sda2 partition
You may ask why that happen. Well could be multiple reasons. One of them was probably that the system did not shutdown normally
I have to tell you that probably you didn't lose anything, was your "tech guy" stupidity and ignorance that caused that. I can't confirm this 100% because you never tried to fix the disk with the tools in hand.
And about the hidden partition, a "tech guy" should know that in a GPT layout installation you will have a hidden partition used for recovery. Is part of the standard and doesn't have any ties with the system you choose to install. Probably that was that "mysterious hidden partition".
You can see that this could happen also on Mac OS, almost identically because it's ties with Unix (same command fsck)
https://www.howtogeek.com/236978/how-to-repair-disk-and-file-system-problems-on-your-mac/
Solution
Follow what it says
- Run
fsck /dev/sda2 (Say yes to everything) or run fsck -fy /dev/sda2 (forcing and saying yes to all automatically)
- Reboot
If the problem persists after you run fsck, that could mean other things, but always go one step at a time.
What you meant with "Crapple kernel"?
