Shut down your Mac. Press the power button, and as soon as you hear the startup chime, hold down Command-Option-P-R. Keep holding down those keys until you hear a second startup chime. Then let go and allow your Mac to continue starting normally. Then check the Startup Disk, Display, and Date & Time panes of System Preferences to make sure theyre set the way you want them.
If you hold down Command-Option-P-R at startup and you see nothing but a gray screen that doesnt change for several minutesno Apple logo, no progress bar, no second startup chimedont panic. The most likely cause is that your Mac isnt registering the key presses due to wonkiness with a USB device. Disconnect all USB devices (except your keyboard, if its a wired keyboard), hold the power button down until the Mac shuts off completely, and then press it again and immediately hold down Command-Option-P-R.
If that doesnt work and youre using an external Bluetooth keyboard, try plugging in a USB keyboard instead. If youre able to reset the NVRAM successfully with the wired keyboard, you can disconnect it and go back to your normal Bluetooth keyboard.
