I did an update for my iMac, and now it won't start up.
MY iMac will not start after a system update
iMac (27-inch, Late 2013), OS X Yosemite (10.10.4)
Try these steps:
Restart your Mac with the Shift button pressed.
- Shut down your Mac
- Wait until your computer turns off and after that press the Power button
- Right after you hear the startup tone, press and hold the Shift key
- Release the Shift key when you see a grey Apple sign and the progress bar below this sign
- Once you see Desktop, start a Disk Utility scan to detect and repair file system errors (don't forget to choose your main hard drive)
- Click on Verify Disc and then, if asked to fix problems, on Repair Disk
- After this, click on Verify Disk Permissions and then on Repair Disk Permissions
- After the process is finished, shut down your Mac and turn it back on after about 30 seconds
Apart from that, take a look at this Apple article and follow the instructions on Resetting NVRAM shown there:How to Reset NVRAM on your Mac - Apple Support
Take a look at the steps suggested by Apple in this article:
https://support.apple.com/en-us/HT204267
Hope this helps!