Q: mac 10.11 ssd question

- 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 are running an SSD, you need to have TRIM enabled. For 10.10.5 and later, you can use this Terminal command:

sudo trimforce enable

Then enter your login password (which is not echoed) and read and agree to the disclaimer, then it will Restart.

If you did not use Apple Trimforce command to previously turn on TRIM (only applicable to 10.10.4 and later) the method cause problems with 10.11.

How did yo previously turn on TRIM?

What OSX version did you update from?

See:

http://www.cindori.org/trim-enabler-and-yosemite/

What OSX version did you update from?

What model Mac Pro?

When did install the SSD? The silver tower Mac Pro desktops have four drive bays and two normally unused SATA connectors on the logic board.

What model SSD?

What specific model EVO? There are many

Some users have reported problems with some EVOs?

Is the firmware on the SSD up to date?

You posted in the Mac pro desktop forum. I requiested you post be moved to the MacBook Pro forum

The flex cable connecting the drive to the logic board is prone to failure if not done carefully when replacing a drive.

If you reinstall the original disk does it work OK?