Q: Computer sleep mode

Try steps in here

If your Mac doesn't sleep or wake when expected - Apple Support

Display sleep time should be less than computer sleep time . a screen shot is attached that will help you .

- 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

• Note the time of the next unintended shutdown. After restarting open the Console app in Finder>Applications>Utilities and look for log entries at the time of the shutdown. Post suspicious ones here.

Look before the log entry that starts with BOOT_TIME since that is the first line of a bootup

- 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