Diagnose your Mac and reset its NVRAM
1 Run a diagnostic
Power on your Mac and hold down d before the grey screen appears until you see Apple Diagnostics or Apple Hardware Test. If this doesn’t work, try a wired keyboard. On some older Macs, you’ll need your original install discs for this.
2 Reset the SMC
Shut down your Mac. It’s wise then to disconnect all peripherals apart from any Ethernet cable and your keyboard and mouse. Unplug the Mac’s power cable, wait 15 seconds, plug the power in again, wait five seconds, then power on.
3 Reset the NVRAM
Get your hands ready to hold down
ç+å+p+R, then start up your Mac. As soon as you hear the startup sound, hold down those keys until you hear the sound a second time, then release them, and allow the Mac to start up as normal.