You’re probably either experiencing a corruption of some sort, or possibly a hardware problem.
Which model is this? (If your footer is the correct information for this question, there probably isn’t a hard disk here as only the oldest MacBook Air had hard disks and those aren’t supported on Mojave. But then this question is posted in the macOS Sierra area?)
For the following sequences, remove all external devices. All hubs, all USB devices, all external storage, everything. Problems with connected hardware and external I/O cables can sometimes mess up the whole environment.
First see if booting in Safe Mode works, as that rebuilds caches and disables various add-on extensions.
If Safe Mode does not work, then try booting Recovery to reload macOS and which is usually harmless if you don’t also wipe the disk or SSD, or wipe the disk or SSD and reload macOS and then restore your backups, but you’re also potentially headed for a repair here. Both hard disks and SSDs both fail, as does other hardware.
If you don’t have current backups, boot Recovery and use Disk Utility and external scratch storage to (try to) get a backup.