Ubuntu 16.04: Can only boot in recovery mode

What happened:

1. Ubuntu 16.04 works fine.
2. Emergency in our building → computer was forced to shut down immediately.
3. Since then: Ubuntu doesn't boot properly:
   • Screen stays gray not black before login screen appears
   • Recovery mode → fsck → Resume boot → Everything works fine

What I found in journalctl boot logs:

/dev/sda2 contains a file system with errors, check forced.
/dev/sda2: Resize inode not valid.
/dev/sda2: UNEXPECTED INCONSISTENCY; RUN fsck MANUALLY.

I guess I need to run:

fsck -a /dev/sda2

Can it harm my system in any way? Do I need to unmount sda2 first?

fdisk -l says:

/dev/sda1      2048    1126399    1124352  549M EFI System
/dev/sda2   1126400    3174399    2048000 1000M BIOS boot
/dev/sda3   3174400   35942399   32768000 15.6G Linux swap
/dev/sda4  35942400 7501475839 7465533440  3.5T Microsoft basic data

Solution 1:

Your filesystem is inconsistent, so you need to run fsck as you write. It's best to unmount /dev/sda2 if possible, or boot from a Live media, and then do fsck.

Will it harm? It will bring your filesystem back to a consistent state, clearing dangling inodes and blocks. You will probably not lose data, unless your disk is severely crashed, but then data is probably lost anyway.

A good backup from yesterday is always a good asset to have.