I woke up this morning to find that my MacBook Air had totally shut itself off overnight. So I powered it up and all I got was a plain blank screen that had a light gray background. Something was wrong. The 1st thing to try when in this situation is to boot your Mac into Safe Mode, which is done by pressing the SHIFT key after you hear the startup chime when powering up your computer. (if you hold down "shift-command-v" it gives you a progress bar for the safe mode boot) Keep holding down shift until you see the apple logo. In my case, my Mac would not boot into safe mode. It's Labor Day today so there is no Apple Care support to call or Genuis Bar appointments to be had. I was on my own. So now what?
Apple has a support article pertaining to this exact situation (the gray screen of death part, not the not having a Mac problem on a Federal Holiday part). The support article recommends (after attempting to boot into safe mode) to reset the NVRAM/PRAM. What is NVRAM and PRAM? Apple has a short description of that here. Basically NVRAM and PRAM are special types of memory used on Intel-based Macs to store certain types of system parameters. Several of those system parameters are used to boot your computer, so if they become corrupted it can cause you boot issues. So I followed the directions to reset the NVRAM and PRAM, which is to hold down the Command-Option-P-R keys before the gray screen appears. Hold down these keys BEFORE you press the power button to turn your Mac on. I know, this takes a bit of finger stretching to accomplish (get a second person to help you if needed). Keep holding down these keys until you hear a 2nd startup chime (the memory is reset in between the 1st startup chime and the 2nd).
Resetting the NVRAM/PRAM did the trick for me. My Mac then booted up as normal and I was off and running. Restting the NVRAM/PRAM is used as a bit of a cure-all in the Mac community. If your Mac is running a bit slow or something seems off...reset your NVRAM/PRAM (at least that is a lot of people's thinking). It's actually a very safe thing to do and can help your Mac run more smoothly in some situations. In my case, it was exactly what I needed. Something within the NVRAM/PRAM memory was keeping my Mac from booting.