Mac peripherals, network, or startup not responsive
Many issues that appear to be hardware are due to one of two circuits on the computer going south.
The SMC, the system management controller may get erroneous power information that leads to it needing to be restarted.
The PRAM, also known as the CMOS, which is sometimes managed by a capacitor, and sometimes by a separate battery on the main board (only known to Apple Service Centers) may need a reset signal sent. This should only be done if the computer is under 4 years old. Older batteries/capacitors when reset have been known to disable the video of the computer.
Both SMC and PRAM can affect a wide variety of settings including built-in hardware settings for WiFi, and any peripheral ports, and sometimes display.
To find the age of the computer, visit http://checkcoverage.apple.com/ Do not give anyone on the support community the serial number as that is personal information. If you call AppleCare directly, only they should know your serial number.
If older than 4 years, reset the SMC and see if that provides the necessary feedback for your computer to work again.
If it is younger the 4 years, reset the PRAM (connected a wired keyboard to your Mac if it is not a laptop, if it is a laptop, use the built-in keyboard), and hold command-option-P-R through a restart (PC keyboards use Windows Logo key for command, and Alt for Option). The machine may chime or a light may blink each time it is reset. If you get light or chime feedback, you can wait up to 4 feedbacks before letting go and starting up.
Find the appropriate forum for your computer or if you know it, your operating system and ask if your situation is not repaired.
If neither of these options work or you don't have a power cord, batteries on board the computer also only last 2-5 years. Batteries - Apple explains in more detail how Lithium Ion batteries behave. The System Report will give you the number of cycles. Apple does offer battery replacement at cost at their authorized service centers.
If you have a power cord, be sure it is the right one for your model:
If your MagSafe cable or power adapter isn't working - Apple Support
Identify your Mac power adapter - Apple Support