I hate to tell you, but most likely the hard drive is failing. There is a slim chance the internal hard drive SATA Cable is bad which is common on the 13" model, but my money is one a bad drive.
Try running the Apple Diagnostics to see if any hardware issues are detected. Unfortunately the diagnostics don't detect a lot of drive failures. If no errors are reported, then a better option is to use a bootable Knoppix Linux USB stick to check the health of the hard drive (it will tell us how bad the failure may be which can determine the next best course of action).
Download a Knoppix v9.x or v8.6.1 DVD .iso file with "EN" in the name for "ENglish". Use the downloaded .iso file as a source for Etcher (Mac/Windows/Linux) in order to create the bootable Knoppix USB stick. Option Boot the Knoppix USB stick and select the orange icon labeled "EFI". The Mac may appear frozen on the Apple boot picker menu while booting, so give Knoppix lots of time to finish booting.
When Knoppix reaches the desktop click the "Start" menu icon on the lower left corner of the Taskbar and navigate the menus to "System Tools --> GSmartControl". Within the GSmartControl app double-click on the laptop's internal hard drive icon to access the drive's health report. Post the complete health report here. I believe there is a tab which allows you to view the recent errors as well which may prove useful in case the main health report is inconclusive.
If the hard drive is failing, then you need to be very careful & selective on how you proceed to recover the data because the more a failing drive is used, the worse the failure will get where even a professional data recovery specialist will be unable to recover any data. macOS, standard apps, and even data recovery apps cannot handle the errors produced by a failing drive so they get tangled up in them which makes the errors and the failure worse which increases the loss of data. At this point checking the health of the hard drive is your best option so you know what you are dealing with. If the drive is healthy, then you will have more options available to you which you can try yourself. Don't use the drive until the health of the drive has been confirmed if you value the data still left on the drive.
You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.
