If you're on OS X 10.9 or higher,
download SMART Utility here. This will verify the SMART data of the SSD. Anything other than passed means the SSD is faulty.
For an older OS, download it here.
If that comes back clean, try to verify the volume in Disk Utility. If the volume is corrupted. please quote this and we will run through a repair within the recovery partition.
Updating the OS is always a good shout. Get onto the latest version of El Capitan (10.11.4) if you're not already. This can be downloaded/updated through the App Store, and clicking the 'Updates' tab.
You can try reset the PRAM. Turn off the Mac. As soon as you press the power button to turn it back on,
press and hold the following keys:
Cmd +
Alt +
P +
R. Continue to hold these keys down until you hear the Apple boot chime 3 times, then let go.