Flip your Macbook Pro so the bottom is facing up.
Look for the model number.
It will be an A with 4 numbers, such as A1425, or A1502.
Which model number is showing?
Next step would be to try booting to your recovery partition. Restart, while holding Command + r
When you get to the menu screen, open the Disk Utility, and select your hard drive, then try First Aid.
If that reports no problem, try reinstalling your macOS system. That's one of the choices on that menu screen.
If that does not help, I suspect that your hard drive is probably failing, and you will probably need to replace your hard drive.
EveryMac.com's Ultimate Mac Lookup - lookup Apple Mac, iPod, iPhone and iPad specs by serial number, order number, model number, model ID, EMC number and more.
everymac.com
Pay attention to this: "Because there are 12 MacBook and MacBook Pro models that match the A1278 model number, you may wish to lookup yours by Model ID or Serial Number for greater precision."
Put in the relevant info and you can narrow down to what specific year it is.
IF it happens to be a 2012 MBP, there was a problem with the internal drive ribbon cable on these.
The cable is cheap and easy to replace... even you can do it (with the right tools, a Phillips #00 driver).
Go to ifixit.com and see how it's done.
Reinstall your system software by booting to the recovery system - Restart, holding Command + r
Choose "Reinstall macOS"
That will ask you to select your existing Mac boot volume. That's the destination for the install, then continue with the install. It will download the installer files, and complete the install. This might take 30 minutes to an hour to complete.