- Try resetting the SMC and NVRAM/PRAM
Intel-based Macs: Resetting the System Management Controller (SMC)
https://support.apple.com/en-us/HT201295
About NVRAM and PRAM
https://support.apple.com/en-us/HT204063
- Try starting in Safe Mode
OS X: What is Safe Boot, Safe Mode?
https://support.apple.com/en-us/HT201262
- Boot to Recovery and repair the startup disk
OS X: About OS X Recovery - Apple Support
https://support.apple.com/en-us/HT201314
- If it is repairable reinstall the OSX
How to reinstall OS X on your Mac - Apple Support
https://support.apple.com/en-us/HT204904
- If you do not have a backup use disk utility to restore the internal disk to an external disk so you can try to recover data.
Then format the startup disk and do a fresh install of OSX
If you can boot to Recovery you can try to copy files to an external drive:
Copying Files to External Hard Drive via Terminal When booted to Recovery?
Alternately you can install OSX on an external drive via Recovery and boot from that and copy file from the problem disk