These store key information for your Mac. Resetting the SMC and NVRAM is no panacea, but can transform an apparently dying Mac.
SMC
All the useful key combinations, arranged by function rather than keys.
What to do when your Mac won’t start up, and is acting dead, or at least mortally wounded.
After an interrupted firmware update, my iMac refuses to start up. What can I do?
The Eclectic Light Company 