Doesn’t start up

OK try this first, Shutdown your Mac. Now start it up but press and hold down the Command and R keys.

The Mac should boot into the Recovery HD, when it does there should be a box with various choices select

Disk Utility, now click on View in the menubar and select Show All Devices, now highlight the Disk in the left hand column and then run First Aid, make sure you get a green tick. now select the indented Disk/ Volume and run First Aid, if there are any errors run it again until all errors are cleared and you get a green tick. Now restart the Mac.