Is the data backed up? May be a macOS issue, or a failing hard drive. Try to boot into Recovery Mode, Command + R on startup and repair the internal startup disk.
MacBook Pro (15-inch, Mid 2012) 2.3 GHz Core i7, 16GB, 256GB M4 SSD
iPhone 8 - 64GB S/G • Sound System Audio Engine A2 • Display UltraSharp U2412M 24"
Custom Built Gaming PC Kaby Lake Edition | 16GB DDR4 | GTX 1070 8G